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
2011
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"
2012
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