You can now use Quartz Composer-based iTunes Visualizers as screen savers really easily. Just make a copy of the visualizer and put it in ~/Library/Screen Savers.

Open up that copy in Quartz Composer (Developer Tools required) and, while in the Editor window, choose Editor » Edit Protocol Conformance. On the new screen that appears, check off the Screen Saver option and save the file.

To have the visualizer actually work as a screen saver, a QTZ visualizer must be running in iTunes.

[robg adds: When I tested this, I was able to get the screen saver to work even when iTunes wasn't running, so I'm not sure about the last bit of the hint. I found one sample Quartz Composer visualizer in the Developer Tools install, on this path: /Developer/Examples/Quartz Composer/Compositions/Music Visualizers. This MacUpdate search shows a few others that match the terms iTunes, visualizer, and Quartz, but I haven't tested them.]