We all know World Of Warcraft is acknowIedged as the most successful MMORPG of the model of paying a flat monthly subscription fee. However, considering "free to play, pay to get extra advantages" model is widely adopted by more and more online games, Blizzard doesn't say they deny the possibility of micro-transaction/free-to-play model for World of warcraft in the future. Probably, they are working on it, according to Videogamer's interview with Tom Chilton, the lead designer for World of Warcraft.

And Tom 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!

Again, they don't give a clear confirmed statement, but I still can imagine how badly it could piss off some WoW junkies, especially those who shits free-to-play games. Anyway, hope it can go well in that micro-transaction way.

You might be interesting with the interview, check here for full of it.
Blizzard's Tom Chilton on the future of WoW