Here is a release schedule like nothing we have been hearing but from another confident source. It makes Tau the next codex, and I can hear the celebrating and cheering from all the Tau fans around the world.

Please remember to take with a grain of salt

via Stickmonkey
My sources are giving me the following general timelines of codex releases
2012
Q1/2 - tau (or flipped w BT)
Q2/3 - BT ( possible WD codex)
Q3 - 6 Ed
Q3/4 - eldar

2013 getting fuzzy
Q1 - SM
Q2/3 - chaos legions (new)
Q4 - ig or orks!

2014. So much could change...no order given
Daemons
Nids
DA?

This is a long way out, and a lot could change, but we know the studio has long lead times.

Cheers, Merry Christmas.