Last year Microsoft contracted Stardock (and Stardock Design) to work with them on an animated wallpaper feature for Windows Vista Ultimate. The feature went on to be called Windows DreamScene which plays video content as animated wallpaper.
Our design team created the default computer generated wallpapers that come with Windows DreamScene. Our development team created a program called Stardock DeskScapes which extends Windows DreamScene to support the new .DREAM format which supports dynamic content (instead of video, generated dynamically) as well as support for triggers and author credit.
WinCustomize.com worked with the Windows Ultimate team to create a community for the uploading and sharing of animated wallpapers in the .DREAM format.
Today, Microsoft made Windows DreamScene Preview available as an Ultimate Extra (you can get it from Windows Update if you have Windows Vista Ultimate). And Stardock has made Stardock DeskScapes available today too as well as a tool to create .DREAM files.
So to summarize, Dream.WinCustomize.com is up which has some new animated wallpapers available. You will need to download DeskScapes (which is free and requires Windows DreamScene to be installed) to use the animated wallpapers.