It was just announced at GDC that we are introducing a new, open distribution service for games created by the community. Games created with the XNA Game Studio will be available over Xbox LIVE. Later today, seven games created by the community will be available for download (for free) over Xbox LIVE. I will have a video up on Inside Xbox as well that shows you how to download the games.
Edit: There are now a few FAQ’s posted over at XNA.com