I don't think MMOs will be a big factor. At least not this generation. Aside from WoW and free PC RPGs like Maplestory or PuzzlePirates (which would probably appeal more to Wii's demographics than Sony/MS) I don't think MMOs have any kind of mass appeal.
They DO have a core market to be sure but prying that market from PCs will be hard and the masses don't have time or inclination to play any game enough to pay a monthly fee to support it.