Microsoft released their XNA Framework 1.0 and XNA
Game Studio Express
today, which allow you to build games for Windows and the Xbox 360. To compile and play your XNA games on the Xbox 360 you will need to join the "XNA Creators Club" (for $49 per 4 months or $99 anually). There you will have access to additional starter kits, features, etc.
I ported the Rocket Commander game to XNA, just for fun .. and it runs great. It is optimized to run on all 3 cores of the Xbox 360 and I will finetune it a bit in the next few days to run perfectly on the XNA 1.0 release. The performance is really good, the current build has over 800 fps in PAL on the Xbox 360, in very high resolutions (have only 1600x1200 monitor, but I will test it on 1920x1080 too later this week) it still runs good with more than enough frames.
Here is a little preview picture, it shows about 8 times more asteroids and runs at over 200 fps here. I will try to post some of the porting problems in the next few days, for example all the network and internet code had to be ripped out from the Xbox 360 build and the sound features are a little stripped down in XNA (no 3d listener support, not easy or not even possible to do any stereo or surround sound, looks like playback is all mono :-( ).
Early alpha screenshot of Rocket Commander XNA: