I just discovered that the + and - keys adjust the iTunes volume. Nice to have an easy keyboard shortcut for this that compliments the global sound volume keys. Oddly, the = key also raises the volume in iTunes.
[robg adds: I could only make this work when iTunes was in its mini window mode; with the full size window, the keys had no effect. The = key probably works so that you don't have to reach for the Shift key to make the + sign.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060811102009424