As many people may know, Apple made it all but impossible to run any screen savers on top of the login window (i.e. after some time, a screen saver appears over the login window) in Leopard by tightening up security requirements on processes that can run over the login window via launchd. This made workarounds that worked prior to 10.5 impossible to use. (The only screensaver that I know of that can run behind the login window in 10.5 is this one.)
I'm happy to report that, as of Snow Leopard, running screen savers behind the login window is finally officially supported. This was brought to my attention by magnusviri in this Apple Discussions thread .
Here is how to make it work...
Use Property List Editor or TextWrangler to create the file /Library » Preferences » com.apple.screensaver.plist with the following content:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>loginWindowIdleTime</key>
<integer>180</integer>
<key>loginWindowModulePath</key>
<string>/System/Library/Screen Savers/Nature Patterns.slideSaver</string>
<key>moduleDict</key>
<dict>
<key>moduleName</key>
<string>Skyrocket</string>
<key>path</key>
<string>System/Library/Screen Savers/Nature Patterns.slideSaver</string>
<key>type</key>
<integer>0</integer>
</dict>
</dict>
</plist>
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100107141643248