"internal operational error".
Microsoft states it is "actively working to resolve the issue as quickly as possible," but has not given any timeline on when the Marketplace will be up. Xbox Live, matchmaking and other online features are unaffected.
This has to be frustrating for Atari, who released Dungeons & Dragons: Daggerdale yesterday on Xbox Live Arcade. Expect Microsoft to give away a free game to users; in 2007, a similar outage resulted in free copies of Undertow as an apology.