Gamerbytes, a downloadable game blog affiliated with Gamasutra, has an interesting report that reveals sales data for Xbox Live Indie Games titles in 2009. The data is heartening, and shows that the service can be lucrative. Hopefully these reports will drive more developers to XBLIG and start a positive cycle that will increase the quality and visibility of these games.
Also, please check out this thread on the XNA developers forums, where much of the data was gathered.