This is my speculation based solely on the correlation of tons of rumors in their entirety.
March/April
Necron, Tyranid, and Space Wolf models
June-August
Chaos Legions
July
6th Edition
Q3-Q4
A mystery codex...... Eldar, Dark Angels, Tau.
Here are the two conflicting rumors. Please take them with a good amount of salt.
via Mr Swaggle
I have some information that I have gathered from a source that is yet to be proven reliable but I thought I would share with the community.
As a dedicated Tau player I was disappointed to find my blue men will not release this year, nor will BT or DA. Evidently there will only be two armies released this year.
From what I "overheard" the summer release is supposed to be Chaos/CSM and Eldar. I know sounds nuts. But the spring is going to be one of those two with the other followed in the winter/fall. Again I do not know how this makes since but it is GW so all is fair.
The reasoning I picked up was that GW wanted two big releases this year to coincide with anniversary. I found it strange though that it wouldnt be marines or orks but heh!
Besides that though the releases are supposed to be the largest to date with something like 80% of the new models released upon arrival.
I also got that 2013 will see Tau and orks, with a huge space marine book. All three of those armies (huge grain of salt) will also be getting a large holiday battalion box.
Also DA will get summer White Dwarfed in 2013 and BT will be in marine book.
Besides that I dont know how much stock I put in these rumors but I thought it was worth bringing up. Also I purposely created this logon just so I wouldnt be harrassed by an angry mob of gamers (we all do it)
via Straightsilver
I may live to regret this, but there will be no Eldar release in 2012. They are still a long way off, at least a year away from what I have been told. of course now I have said that I will probably be proved wrong, but I am 99% sure they won't be this year.
Tau on the other hand are a strong contender, although they have had models ready to go for over a year and still haven't shown up.
2012 looks like this as far as I know:
February: Nids Wave
March: Necron Wave
June: Chaos Legions
July: 6th Edition boxed ste (featuring Dark Angels and Chaos)
August: Codex Dark Angels
There are no signs of any other Codex releases, there may be waves for other armies.
Q4 this year will be dominated by the Hobbit so expect Q2 and Q3 to be 40K centric, but don't expect too much.
Then Q1 2013 will be more Hobbit, Q2 2013 may see a new Codex but it's more likely to be wave releases, so if Eldar or Tau were coming I would say Q3.
Of course I may be wrong, but these are the indications I have been told.
Then again, one of those sources said Tau were due before Necrons, so who knows....