If you hold down shift while moving your mouse's scroll-wheel in an iTunes window, the volume slider will move up when you scroll down (and vice versa), and not as expected. Weird, huh?
[robg adds: We covered the "normal" behavior of the slider via scroll wheel in this hint. iTunes must be the foreground app for this trick to work, and the behavior differs by window size. If you have the iTunes window in its most-shrunken state, the mouse just has to be over the window somewhere. If you use a larger iTunes window, you'll need to put the mouse directly over the volume slider to make this work.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060506213915679