After the release of the 1.0.8 patch for Diablo 3, some players uncovered a bug that allowed them to duplicate gold through both auction houses and gather billions--and in some cases, trillions--in ill-gotten gains, forcing Blizzard to close the AHs.
The bug has supposedly been fixed and is being tested, and the AHs will be back online once the fix is live. However, Blizzard has decided not to rollback the servers to pre-patch status, instead choosing to deal with the individual player accounts that decided to play fast and loose with the rules, namely don't exploit bugs you find.
"At this time (and after careful consideration), we've decided to not move forward with rolling back the servers. We feel that this is the best course of action given the nature of the dupe, how relatively few players used it, and the fact that its effects were fairly limited within the region," Blizzard explained on its forums. "We've been able to successfully identify players who duplicated gold by using this specific bug, and are focusing on these accounts to make corrections. While this is a time-consuming and very detailed process, we believe it's the most appropriate choice given the circumstances. We know that some of you may disagree, but we feel that performing a full roll back would impact the community in an even greater way, as it would require significant downtime as well as revert the progress legitimate players have made since patch 1.0.8 was released this morning."
The bug supposed resulted from Blizzard lowering the cost of gold on the real-money auction house from 25 cents for 1 million to 25 cents for 10 million. Apparently, when players canceled a gold auction, they would be refunded twice the amount listed.