Quartz Compositions as iTunes Visualizers
Chris Vogelbusch has created iVisualize, an iTunes plugin that allows any Quartz Composition to run as an iTunes Visualizer.  An iTunes version of Soundstream comes bundled with iVisualize.
After you install iVisualize, you can click the Options button and select iVestream.
Soundstream in iTunes
System Requirements:
  1. PowerPC or Intel Macintosh
  2. Mac OS X Tiger 10.4.4
  3. Quartz Extreme-compatible graphics card