Blizzard has temporarily disabled the Diablo III auction house after a new exploit was found that allowed users to duplicate gold. The exploit was introduced with the recent patch 1.0.8 which dropped early yesterday. Reports on the battle.net forums have said at least one player has generated 371 trillion gold using this duping exploit.
No one has been reported making real world dollars using their in-game gold just yet. While you can purchase gold using real-world money, there is no easy way to exchange for real-world currency short of selling equipment on the auction house that Blizzard has disabled.
Blizzard has already found a fix to the problem and has recently implemented it. They will not, however, be rolling back the servers to a pre-patch state. Instead, they have decided to individually identify the players who used this glitch and deal with them on a case by case basis.
Source: Battle.net