Mac OS X Hints has an article which explains how to run a screensave module as your desktop wallpaper! Just open Terminal (Applications:Utilities:Terminal) and type the following (make sure to remove the line break):
/System/Library/Frameworks/ScreenSaver.framework/Resources/ScreenSaverEngine.app/
Contents/MacOS/ScreenSaverEngine -background
This launches your currently selected screensaver. To explicitly select a different one, use -module:
/System/Library/Frameworks/ScreenSaver.framework/Resources/ScreenSaverEngine.app/
Contents/MacOS/ScreenSaverEngine -background -module Flurry