These are rumors, please add the normal salt.
via Lord Commissar Aquila
Ok, so I was talking with someone who has connections to one of the model designers, and this is what he told me:
1. The Eldar dex will be out right before the release of 6th edition. Phil Kelly is the writer, which is why he wasn't able to write up the Cron codex.
2. 6th Ed will be released in June or July of next year (2012) if everything goes to plan; its currently in playtesting right now.
3. The starter set might be the Fall of Damnos, but he said he wasn't completely sure it would be set there. It will include Space Marines, of course.
4. CSM will get one of the newest dexes in 6th, right after the Marine dex.\
5. Tau and BT will get a release soon after CSM; other races updated in 5th will not be updated in 6th as their rules (at least from Guard onwards) are allready suited for 6th. Ward will probably do the BT and might do Tau as well. DA will probably not be updated anytime soon, because its not a popular army and GW does not want to waste money updating one or more armies that almost no one plays with a separate dex.
via brassangel
Most sources say that Tau are coming before 6th (Q1 2012), Chaos Legions will be the first 6th edition book, and the new edition is slated for the weekend of July 14th 2012.
via MajorWesJanson
From Harry and other rumor sources, I'm far more inclined to believe that we will see BT in March/April, Tau in May, 6th in July, new starter (Chaos vs someone, Eldar?) in August/September, and one more codex in Oct/nov, likely Chaos Marines/Legions. Eldar after that.
I'm also more inclined to believe either Kelly or Cruddace did Black Templars and the other did Tau, and Ward will be doing CSM.
Odds are against Kelly doing Eldar, as he did not really want to redo it as he wrote the last one. If Ward does CSM, that leaves Cruddace open to do them.
CSM are likely to be the first dex of 6th, as there has been no real rumblings of a new marine book. With the supposed "Chaos focus" of 6th, having CSM/Legions get the first hardcover dex would fit. Then Eldar, with a history of fighting necrons and now mortal enemies of Chaos.