Microsoft XNA Game Studio Express 1.0 is available now! The game launcher is available via Xbox 360 Marketplace (called "XNA Creators Club")!

Download the studio from here. (thanks to SuperSpit for the link)

http://www.msdn.com/xna/creators/


Notes for Vista install
Make sure you have the VS 2005 SP1 Beta installed (you can get it via http://connect.microsoft.com/). After installing that I was able to install the XNA Game Studio Express 1.0 without any problems in Vista.
I have also run the Space Wars template without any problems. To avoid most possibly problems right click the XNA Game Studio Expression shortcut and click "Run as administrator".