we got reports (thanks Mike, Matt...) that Microsoft is having issues with its WGA validation servers. This may affect downloading some software from Microsoft as well as new Vista installs. From posts to a Microsoft forum, it looks like this may persist for a few days. I am not sure if the phone-based activation still works, but its something you may want to try.

This should mostly effect Vista users.

For the Microsoft forum see:

forums.microsoft.com/Genuine/ShowForum.aspx

and a statement from MSFT: forums.microsoft.com/Genuine/ShowPost.aspx

UPDATE

The forum is stating that the issue has now been resolved.  With an explanation forthcoming.