Blizzard has introduced a method for trading real cash for World of Warcraft gold, albeit in a roundabout way. As noted on Rock, Paper, Shotgun, a new Guardian Cub is to be offered as a cosmetic pet in the MMORPG, but unlike similar items in the game, this does not bind to your account unless you choose to use it, meaning you can buy it for cash and trade it or sell it at the auction house for whatever amount of gold it will bring. In the related FAQ, they say they are comfortable with the economic implications of all this:

While our goal is to offer players alternative ways to add a Pet Store pet to their collection, we’re ok with it if some players choose to use the Guardian Cub as a safe and secure way to try to acquire a little extra in-game gold without turning to third-party gold-selling services. However, please keep in mind that there's never any guarantee that someone will purchase what you put up for sale in the auction house, or how much they'll pay for it. Also, it’s important to note that we take a firm stance against buying gold from outside sources because in most cases, the gold these companies offer has been stolen from compromised accounts. (You can read more about our stance here.) While some players might be able to acquire some extra gold by putting the Guardian Cub in the auction house, that’s preferable to players contributing to the gold-selling “black market” and account theft.