Speaking here, Blizzard's Rob Pardo has said that WoW players want a fair financial system once they're in the game and that micro-transactions would "betray" them.
"We chose to go with the subscription-based model instead of that approach," he said. "We've taken the approach that we want players to feel like it's a level playing field once they're in WoW. Outside resources don't play into it - no gold buying, etc. We take a hard line stance against it. What you get out of micro-transactions is kind of the same thing and I think our player base would feel betrayed by it. I think that's something else you have to decide on up-front instead of implementing later."
Blizzard is currently working towards the release of Wrath of the Lich King, the second major expansion to its 10-million strong MMO.