With the iTunes visualizer enabled, press the Q or W keys to move back and forth through a list of visualizer configurations. The A and S keys let you control the basic waveform being used to seed the visualization. The Z and X keys cycle through color palettes. Press the C key to display the current configuration.
Use of the ? key to control basic config (framerate, etc.) is documented, but as far as I can tell, the hotkeys above are not.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20011024000833910