There are a few options from the quick search i just did: http://mac.softpedia.com/get/Screensavers/bgscreensaver.shtml
With that program, you can make a screensaver using core animation (for example), then run that screensaver as your desktop. I don't have leopard yet, but I understand this is supported without this hack. This could be misinformation, though.
It looks like you can set up any video that quicktime can play to use as your desktop. The screensaver thing seems more CPU efficient, but... this is all fluff, so whatever you do is burning CPU for eye candy anyway.