XNA Game Studio Express provides a set of cross-platform libraries and development tools for creating games for Windows XP, Vista and the XBOX 360 console. The current beta is targeted for Windows only, and requires Windows XP SP2 according to the official Microsoft requirements. The current beta allows for development of PC games only, but the final release, scheduled for a release “this holiday season” will allow developers to join the “XNA Creator’s Club” for a $99/year membership fee.
I look forward to seeing what becomes of this… it should prove quite fun after a few months when many homebrews are available. The beta includes source code and a compiled version of a game called Spacewar to get you started.