Windows Vienna is the codename for the operating system that comes after Microsoft's Windows Vista. I'm not yet sure why Microsoft is worrying about the OS after Vista, when Vista isn't anywhere near getting out the door. This information comes from Neowin.net where they discuss how this info was discovered from famous Microsoft blogger, Robert Scoble. Apparently Scoble replied to a thread on Channel 9, an MSDN related site, regarding a Blackcomb video with the following:

since this video was shot the codename for Blackcomb has indeed changed to Vienna. I haven't gotten the story on this yet, though.

Windows Vienna used to be codename Blackcomb. Microsoft is still remaining fairly secretive about Vienna and I don't blame them. Vienna probably won't come out until 2010.

While Windows Vista is intended to be a technologies-based release, with some UI changes (in the form of the Aero set of technologies and guidelines), Vienna is targeted directly at revolutionizing the way we interact with our home and office PCs.