I've been playing around with XNA a little bit in my free time.  It's pretty darn easy to build a simple game with.  You can debug it from your Windows machine while running on your Xbox 360, all good stuff!

http://learnxna.com/archive/2006/12/11/Its-official.aspx

Be sure to check out the XNA content on Channel 9 too.

P.S. Don't mind the typo on Learn XNA.  "not" should be "now".  😉