I had posted a while ago looking to be able to do this. Now that 10.2 is out it works (and just too slick)
Two programs that can do this (and then add to login items)
Desktop Effects
http://www.versiontracker.com/morein...d=15957&db=mac
BackgroundSaver
http://www.versiontracker.com/morein...d=15989&db=mac
Carl.