And to piggyback on what Tonuss was saying, I don't think another subscription-based MMO will ever surpass the 12 million that WoW had at it's peak. If any game were to ever overtake it, it would be more because of WoW's dwindling population and not some other game just getting that high number of subs.

I think Rift was the first REAL contender, but will eventually go the way of a game like Aion. I think the next real challenger will be The Old Republic, but it probably won't ever have more than a million subs or so.