Here is an updated release schedule for what may be coming out. Most of it is very similar to what we have been seeing since the beginning of the year, so it seems likely that 3 codexs are due on the very near horizon. At the beginning of the year we had predicted the following- Necrons (Sept), Sisters of Battle (End of the Year), and Tau were then discovered to be early 2012.
The following rumors are similar with more alternative options, and the mentioning of a possible name for our summer of fliers, Wartorn Skies. Personally I like the name, and cannot wait to see it. Will I play it, who knows. I will at least look at it, and read through the rules as I normally do with 40k expansions. Its also possible it will be a White Dwarf release, and in that case, I will pick up that White Dwarf.
Onto the rumored Release Schedule (and note it leaves out a possible tyranid 2nd wave this fall)
1 Year Prognosis via MadCowCrazy
May : Tomb Kings 100%
June : Dark Eldar 2nd Wave 100%
July : Fantasy or LotR (Ogre Kingdom?)
August : Summer of Fliers (Called : Wartorn Skies?) 50%
September : Fantasy or LotR, Plastic Tau Pathfinders/Vespids?
October : Necrons or Sisters of Battle
November : Fantasy, LotR, Necrons or Sisters of Battle
December : Nothing as usual
Q3-Q4 : Tau are rumoured to receive plastic Pathfinders and Vespids but without a new codex that does not really make sense.
Based on the months remaining for 2011 there seems to be room for only 1 more 40k codex this year
Bindi Baji : Necron September release/"Before October"
January : If Q4 Release was Necrons put Sisters of Battle here
Q1 : Depending on who got the 40k winter release we should see either Necrons, Sisters of Battle and/or Tau Q1
Q2 or Q3 : 40K 6th Edition