WoW gives people less to do on the toilet for a little while.
According to a blue post over on the official World of Warcraft forums and an additional entry on the WoW main page, Blizzard has become aware of unauthorized account activity on player accounts that do not use an authenticator. Specifically, these account owners have reported that they logged in to find all of their gold missing. The post indicates that Blizzard is aware of the problem and they have shut down the mobile auction house until the problem is resolved.
World of Warcraft, like most MMOs, has been at war with real-money trading (RMT) activity since its inception. At various times, companies looking to steal accounts have purchased aggressive ads on popular WoW sites in an attempt to gain key strokes for account access. Please refer to this Blizzard FAQ for information on how to remove or at least protect your account from unauthorized access.
If you suspect that your account may have been tampered with, Blizzard asks that you contact Battle.net immediately. If your Battle.net account doesn't include an authenticator, you might consider getting one over at the Battle.Net services page.