Running a screensaver as a desktop, which this hint explains how to do for Jaguar, works for Panther too. However, Expose will not operate when the screensaver is running, even if it is running as a desktop.
[robg adds: Hmmm, can you visualize how cool a miniaturized flurry screensaver would look, darting about behind 50 minimized windows? Heh...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031024094120407