If you go poking through the Quartz Composer app that tags along with Tiger's dev-tools, you'll find a way to make sound [or keyboard input, or MIDI, or mouse coordinates] the numerical controller on a certain graphical transformation/controller.
Given that some of the tutorials that come with QC are 10.4 screensavers themselves [RSS Display, Colour Field, etc.], it probably wouldn't be much work to rig something up that looked and felt very similar to the screensaver it was based on, but with a more reactionary audio-input component.
As to getting THAT into iTunes, I'm absolutely stumped.