I did that once took me three days to render the movie though before I stuck it as my background. but in a less notable point vlc player will do that to any movie you have
But some of the Graphics on Macs... even I like the Graphics on the Microsoft Software done for Macs. Hopefully they'll do stuff like it for Longhorn Programs.
download a program that converts a video file into a .gif file and you can use the .gif as your background ...but then your computer slows down. i did it once and it took like a whole minute to open desktop properties again lol.