Did a quick search on the forum but didn't see a mention of this yet, and I just stumbled across it...
There is a new site specific for Windows Vista Ultimate:
http://windowsultimate.com/
One of the 3 Ultimate Extras described so far is "Windows DreamScene" which enables the use of .WMV or .MPG files as your desktop background:
http://windowsultimate.com/blogs/ext...reamscene.aspx