This afternoon, we posted the RC1 build of the Windows Media Center SDK for Windows Vista (build 5600.16384) on the Microsoft Download Center. That means that anyone can download it and try it out - it is not restricted to members of the Windows Vista beta program. You can download it from this location:
We will be updating the link on the MSDN home page for Windows Media Center in the near future to point to this download location as well.
Please let us know any feedback you have about the RC1 build of the Windows Media Center SDK and also ask any questions you might have at the Media Center Sandbox discussion forums.