Look at the huge hit wow took as aion was released. And aion was a piece of shit and a korean based mmo.
Now think about Rift, (much better than aion) SWTOR and Guild Wars 2 all being released in the same year i think we are going to see wow hit 6 million subs and stay there for a while. No single game can ever " kill" wow but 3 mmos in a short period of time can slap it in the face pretty hard.
If these games hold up and add some good end game the mmo market could be interesting this year.