Though the two are slightly different elements of in-game play, Blizzard has suggested a real-money transaction addition in World of Warcraft despite their less-than-stellar performance with real-money-trading in their other large-scale online game Diablo 3. This new system is said by one single developer on Blizzard's team - alone in a forum, believe or not - to be part of an exploration of possibilities at the moment, and not quite solid as such.
Though the full system may never make it to the public iteration of the game, testing said system requires that some current in-game elements be tagged with mention of abilities. In this case it was a discovery by an intrepid player and seeker of behind-the-scenes answers that discovered it. Combined with a forum mention of a 100% XP boost in a spell appearing on the game's public test server, here resulting in the following response from Blizzard:
"We are currently exploring the possibility of adding a way for players in certain regions to make purchases directly within the game. As part of this process, elements related to this will be appearing on the PTR. We’ll provide additional updates on our plans as development progresses." - Community Manager Zarhym for Blizzard
At the moment there's been just one similar system in place for the World of Warcraft experience, that being the "World of Warcraft shop" which allows purchase of oddities galore, but outside the game itself. The difference here with an in-game system is the ability to hit the "buy" button without leaving gameplay.
Meanwhile Diablo 3 has a system in place where real-money transactions can take place between players - and they've had their fair share of regrets. Though the systems would seem a bit different on the surface, both would have cash moving in through Blizzard first and foremost - so it's instantly clear why the company would want to roll this all through major tests before any final unveil.
VIA MMO Champion