Here's my attempt at a round up of the last year of Tau rumours. Apologies to those who have had their rumours previously summarised into round ups and thus aren't credited. You have our thanks none the less!

From Stick monkey, September last year.
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New Heavy support platform. Basically immobile deep striked! heavy weapon and crew. Rail and Ion Cannon, and missile launcher are all obvious options here, but nothing solid with the rumor.

Assault Skimmer. Something between size of piranha and hammerhead, fast skimmer open topped, assault on disembark.
Update: Yes, its assault after a flat out move, and has the "drop troops along path" option similar to the storm raven. its supposedly a dedicated transport option for one new unit. (I dont place faith in this last bit, its very rare in current GW systems to see a vehicle kit limited to one unit/role...if GW is trying to sell them they will be available to more than one unit, or be a separate entry unto itself.)

New Drop ship. Not the FW one, but something new. Holds a boatload of fire warriors, or a few battle suits.

New battle suits. Not sure if these are the "next gen" suits or just heavy suits redone with out metal bits.
Also Cypher on Heresy compiled a round-up of the Tau rumours as of April this year

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Penciled in release date of August 2011

All current metal sets to be plastic (Pathfinders, Krootox, Vespids). New plastics are completed for Crisis Suits, Stingwings and Pathfinders.

Plastic Barracuda as well as a new unit choice, which will be a new alien race (insert your fantasy here).

Battle Suits - plug and play system. Essentially streamlined. Pick two guns and a skill or two skills and a gun. New models with interchangeable arms similar to Killa Kans?

Rule Changes

HQ and Troops
HQ choices are used to unlock units for troops, so a commander in a Crisis Suit unlocks Crisis Suits as troops, an Etheral unlocks Fire Warriors, Shaper unlocks Kroot.

Crisis Suits
Mostly same stats but now 3 wounds, come with every gun option and you can choose what weapon systems you fire every round. Max squad size x 5

Broadsides
Can take Network Markerlights instead of the missiles on their arms

Hammerhead Railgun
Draws a line across the table hitting everything under it. Multiple penetration through vehicles, only stopped by a glancing hit.

Shield Generator
Only team leaders can take a shield generator. The shield generator functions exactly like the DE Shadowfield. 2+ invulnerable save until the first failed save at which point it ceases to work.

Drones
No longer bought as Wargear. They are bought as a unit and then divided as you see fit at the start of the match. You can mix your drones when bought for the point cost needed. This caters for Drone Squadrons as an option.

Markerlight
One markerlight can fire the seekers off one model. Seekers replenish and are no longer one use items. Skyrays will only be able to fire one seeker per round.

Also...from BoLS (April 1st - so may just be a wind up!)

1. Kroot HQ is in the works!
2. Kroot to get own transport (Devilfish will no longer be able to transport Kroot)
3. Kroot will get light armor saves (6+)
4. Kroot shaper will allow for the unit to have a special genetic trait.
5. Firewarrior carbine to get a grenade launcher (either explosive or EMP)
6. Vespids reworked and getting a Heavy unit
7. Firewarrior BS increased to 4
8.Crisis suits will get “hit and run” ability
9. Various changes to Firewarrior equipment
10. Various changes to Crisis suit equipment
11. New Prototypes
12. 4th edition prototypes now “standard” equipment for all suits
13. All HQ’s can act as a commander
14. New type of “Heavy” Battlesuit (new prototype of the Broadside)
15. Stealth suits will be changed to a advanced scout unit similar to Pathfinders
16. Pathfinders will be changed to act as a type of Tau Commando infantry
Now on to Ghost's rumours, which primarily cover new non-Tau models. Because there's so many, I've split them up and tried to connect the dots between the hints and match them against each other - my thoughts and interpretations are in italics

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General Codex rumours
Not the next book, but probably after Black Templars

Author may be Phil Kelly

Army has 4 basic HQs and 3 basic troop choices excluding changes made by any Special Characters if there are any

Return of 4 old SC, plus 2-4 new ones

No Court of the Archon/henchmen type mixed units

New Tau models
New SC "Iceheart"

Crisis suits have been completely redesigned (not just recut sprues and ankle tweaks), less boxy and including a 'morphic weapon'. The morphic weapon is actually different ammo types according to Calmsword's source

New sniper suit

Shas'O get a stealth armour option, may be an experimental suit like Shadowsuns, or the experimental suit might be a third option

Allies and Auxiliaries

Vespids get two slots (unknown if this is an additional two, or two overall). Kroot get an additional unit

No Knarloc riders,though perhaps something big (new WFB monster-szied) for the Kroot

Maybe minor psychic powers for Shapers or Shaper like character this is something Ghost has heard from other people and not stated directly

Demiurg have 1 HQ, 1 troop and maybe 1 else. Presumably the Elite choice mentioned elsewhere

Demiurg ancients are a thing Probably HQ, but could also be the elite unit

Possible Demiurg elite choice

Demiurg are "equal to banshees without the scream" so presumably anti-armour close combat, or at least have that as an option. This might also be elite unit rather than the troop one, or be a unit that does both - like a kit that would let you make Boyz and Ard Boyz (or Dwarfs and Longbeards). An elite unit, but being an upgrade to a troop slot to keep out the way of the battlesuits

Demiurg and one other new race join the army, the latter being something bizarre or at least something we've not seen before.

No Gue'vesa unit or models

No Nicassar
Updates!
4 box sets, and 4 blisters in wave 1

Demiurg plastic box
Battlesuit plastic box
Vespid plastic box
Assault transport(?)

Demiurg ancient blister
New Tau SC blister

Demiurg are apparently a lot less Dwarfish than the initial concepts, instead being more like little stone golems (Ozruk from WoW was mentioned)
As a tongue in cheek nod to the fate of the squats, there is a named Demiurg (who may or may not make it as a SC) who survived fighting Tyranids and personally killed a large monster by drilling his way out after being "Swallowed up by the 'Nids"
This Demiurg is aware that the Ethereals have some kind of power over the Tau (maybe other races too) but not the Demiurg, who have presumably signed up for political reasons and of their own free will

Stealth suit mechanics may change
Kroot fieldcraft rule - presumably switched to Stealth and Infiltrate USR

4 new 'Tau' units*: Note these will not all probably be in the first release - there's 2-3 plastic kits here and we already know 3 for the 4 that are coming out.
Dropship (carries upto 15 models or 2 battlesuits and 10 models, or 5 battlesuits)
Fighter
Snipersuits
"Ubersuit"

The Ubersuit is a Battletech/Mechwarrior sort of thing (probably Dreadknight sized), with a special 'ridiculous' ability that lets it rain down shooting if it stands still.
Looks a bit like one of the Battletech Mechs that is named after an ancient weapon

Some units have moved around in the Force Org chart

Posts about Demiurg and Tau special characters apparently need to meet Warseer's 10 character limit, whether posts about other units also need to remains unconfirmed

Thanks to Wyoming Fox, we have some new rumours from Jared Van Kell of Librarian Online
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The Tau codex belongs to Cruddace, lets get that fact straight. That is all I am going to say on that issue.
Black Templars on the other hand was always going to be Phil Kelly's pet project. The only way Ward or Cruddace would ever get the Black Templars codex off him would be to prise the rough drafts out from his cold...dead...hands.
Unless of course they offered him the Dwarf Army Book...............(whistles innocently).

Some of these rumours [WF- In regards to rumor sumary posted up on BOLS - most of which were taken from Warseer] are spot on others not so spot on. 4 generic HQs are the Tau Commander, Tau Ethereal, Kroot Master Shaper and Demiurg Ancient. Blisters are, as far as I am aware, a new Tau Ethereal, Demiurg Ancient and 2 special character blisters.

I've also heard that the Skyray will be getting a better choice of missiles in addition to the Seeker missile which is believed to be S6 AP4 Blast, however you have to choose the missile type at the beginning of the game so you cannot mix and match. Despite the payload ts main role will still be target marking.

The Demiurg Special character is apparently the one called Iceheart.

The Battletech Mech in question is the Warhammer IIC mech (no joke about the name.) one of the unseen mechs in the battletech universe. The suit itself is quite big, not quite dreadknight sized but it can "brace".....yes Matt Ward does play Spacemarine. The trouble is that it will be competing against Broadsides, Skyrays and Hammerheads, all of which have improved, a lot.

Regarding the "HQ unlocks troops" rumour - Well from what I can understand from the mechanic is this. A particular troop type is associated with a particular HQ but in doing so confers a special ability on those troop types.
Tau Ethereal - Unlocks Tau Firewarrior teams.
Tau Fire Commander - Unlocks Tau XV8 Crisis Battlesuit teams.
Kroot Shaper - Unlocks Kroot Carnivore squads.
Demiurg Ancient - Unlocks Demiurg Warrior squads.

I can understand the thought process behind the mechanic from what I know of the basic fluff regarding the Tau.
However I would have thought that Firewarriors would have always remained as basic troops and an Ethereal would have unlocked Honour Guard but it seems from the snippets of info I've been able to peice together that an Ethereal confers Preferred Enemy (Everything) and BS4 on those Firewarrior squads he leads making Honour Guard redundant.

Jared then later added this clarification about the above:
From what I understand they become troops. However I have a strong feeling that firewarriors are going to be remaining as troops that can be taken irrespective of what commander you taken but that the different commanders allow you to take other units as troops instead.
With regard to ethereals I believe that they will allow honour guard to be be taken as troops (with BS4 and preferred enemy) or at least allow firewarriors to be upgraded to honour guard.
For the kroot carnivor squads and demiurg warrior squads and indeed the other kroot/demiurg units as well I do believe the rumour is correct in that you will have to take a shaper/ancient to be able to take them. For Crisis battlesuit teams I am sure taking a tau commander they are moved from elites to troops instead or at least become scoring.

All that I can say is known about the Demiurg is that they are the Tau's main anti-MEQ unit. How effective they are I cannot say.

Actually you might be surprised. My sources and the several other rumour mongers have started indicating that Templars might actually be next followed by Tau, this is normally a sign for me that what I am hearing is correct but with recent developments such things are by no means certain.
I have however recently discovered that the Tau are in the proofreading stage of development which means that the codex is written but it has not yet quite gone to the printers. Now this means that all GW have to do is give the nod and the codex can be printed in numbers pretty quickly. All they then have to do is ramp up the advertisement, distribution and product support and we have them ready to go for our very much enjoyment.

We have another snippet from Ghost about the other new alien race who don't look like they'll be appearing at release; Not Zoats (despite them coming back to WFB in Storm of Magic) but they may be 6-limbed.

Чаще всего подтверждаются новые гномы 40к - Демеурги!!!
По беку тут есть два типа слухов.
1. На самом деле Империей тау управляют они и у них свои далеко идущие и не понятные планы.
2. Для того что бы выжить они добровольно заключили что ни наесть настоящий союз с тау, и мечтают получить мирок другой по теплее в империи.
Ну то что они древнее эльдар это я говорить не буду и так ястно.
По минькам не понятно. Говорят что будет очень кусучий ХтХ, ту также говорял что будет нечто вроде:
1. Нечто вроде баньшей, но без масок и со своими приколами.
2. Или термосы с 4++ и топорами.
Так же ходят слухи о стрелючем отряде, но тут ниче не понятно.
Обещают им одного именого в HQ, сами их отряды вроде как элита, а он их либо усиливает либо переводит в трупсу.

Так же мелькали какието кентавры-наемники с 4 руками. Но про них кроме 2-х артов ничего больше.

Крутам обещают свой транспор, именного (HQ?, абгрейд?), так же новые отряды круты снайперы и еще кто-то (наездника на кларноках? крылатые круты?)

Веспедов переведут в пластик и в хеви сапорд дадут ихний юнит.

Никсар и гуе"веса вроде как не будет

Армии обещают 4 балистик чуть ли не поголовно.
Что-то переделают с системой маркеров. Толи изменят эффекты от них, толи сделают их от количества используемых.
В зависимости от выбора HQ будут открываться те или иные юниты.
Кризисам обещают 3 вунды и возможность их брать до 5. Вроде как им хотят дать хит анд ран по базе но это врят ли.
Дроны или как старые, или берутся на армию, а потом перед игрой распределяются игроком.
Переделают правила на стелсов.
Правило крутов мастера природной жизни станет нечто вроде смеси стелса и инфильтры или будет полностью заменено на них. Так же им дадут 6+ по базе.

Говорят что в первом релизе будет 2-3 пластиковых набора.
Обещали пластиковых пасов, так же мелькали их фотки.
Так же переделают кризисы, и вроде как они будут более округлые.
Говорят о переводе коробок с металом в пластик (веспеды, бронсайды и тд.)
Из новых моделей тау дадут:
Dropship перевозит 15 пехотных моделей и 2 батлсьюта или 10 и 5.
Snipersuits
Так же обещают какого-то ОЧБР, будет выглядит как смесь дредкнайта с мех вориором, должен быть очень стрелючь.
И истребитель (скорее всего Барракуду, но давно мелькали слухи о пластиковых Ремора-дронах)
Так же выпустят побольше всего крутам, но что конкретно не ясно.
Вообще обещали по больше ксеносов, но то как у них с минями (пластик, фаинкаст) будет не ясно, так что про них отдельно.