Quote what the lead designer for WoW - Tom Chilton said:
I certainly think it's possible that we could do some kind of micro-transaction stuff. Whether or not World of Warcraft ever goes the direction of, I guess like Anarchy Online has gone the direction of going free-to-play with micro-transactions. Whether we ever shift to a free-to-play model is really too hard to say at this point. Anything I say now could easily five years from now end up seeming like, oh my gosh, that was an incredibly dumb thing to say, how naive!