Command-Option-] (right square bracket)
This is helpful for when you want iTunes to show you those podcasts that you've deleted, but are still on the RSS feed. If there's a way to do it on a per-podcast basis, I couldn't find it, but maybe someone will. Also, not nearly as useful, but also undocumented ... to make the iTunes volume slider jump all the way down or up (this doesn't affect mute/unmute), hit...
Command-Shift-Down Arrow (for zero volume) or Command-Shift-Up Arrow (for full volume)
Lastly, if you're playing, the following key combo will stop iTunes. If you're not playing and you press it, it will reset the position slider to 0:00:
Command-Option-Period key on numeric keypad (on a PowerBook, add in the Fn key, too).
[robg adds: I believe none of these are documented, but I may be wrong...]

