With FilmLoop you can "drag and drop photos to the desktops of your entire social network in a live, continually updating Loop."
Sounds like they'll have a Mac version at Macworld. It's Windows only right now.
I just heard about it in two places:
Guy Kawasaki has some FilmLoops on his new blog.
Dave Winer informs us that his sister-in-law, Alice Lankester, is VP-Marketing at FilmLoop.