Blizzard has confirmed that yes, an in-game microtransaction store is in the works for World of Warcraft.

"First, we’ll be testing the in-game store with some new kinds of items we’re looking into introducing (in Asian regions, at the outset) based on player feedback: specifically, an experience buff to assist with the leveling process, as well as an alternate way to acquire Lesser Charms of Good Fortune. We’ve had a lot of requests from players in different regions for convenience-oriented items such as these, and as with other new ideas we’ve introduced as WoW has evolved—including Pet Store pets, mounts, and more—your feedback plays a hugely important part in determining what we add to the game."