Community manager, Bashiok, has confirmed on the official World of Warcraft forums that an in-game micro-transaction store will be implemented in the future. The successful MMORPG where subscribers pay $14.95 a month to play has had purchasable items outside of the game but they have stated that they will soon make mounts, pets, leveling buffs, and crafting materials available for real money inside of the game.
Bashiok believes this will be convenient for players so they do not have to exit the game to buy the items they want.
The in-game store is still currently being tested but Blizzard has stated that they will be introducing the store to Asian regions first.
Source: World of Warcraft Forums