Update: The official press release has more details on the program and a few of the first available games. It also mentions that community-created games "will quickly double the size of the Xbox 360 game library."
At Microsoft's GDC 2008 keynote address, the company announced a "community arcade" allowing user-created games to be distributed to Xbox 360 owners worldwide. "Now, 10 million people on Xbox Live get to play your game," said Microsoft Game Developer Group General Manager Chris Satchell. Free trials of these community created games should be available on Xbox Live "immediately" according to Satchell.
"For the first time, community games will be distributed through Xbox Live," Satchell said. "'Xbox Live Community Games will give creators a huge audience to share their creativity with. Game distribution will be democratized, allowing the community to control the content. Create, Submit, Peer Review, Play are the four key steps ... We want creativity to flow through this pipeline.
Users will be able to upload games using a Gamertag-like "creator identity" which will keep track of your game creation history. Creators can use descriptive sliders to describe a game's content, and peer reviewers can also evaluate the content for appropriateness.
At the keynote, Satchell showed off a short demo of user-created game Jellycar (pictured below), featuring an adorable bouncy, 2D car that drives along on a sketchpad, bouncing up and down on a cartoon road.
More as it becomes available.