After reading the hint where the scroll wheel can change the volume in iTunes, I noticed that when iTunes is zoomed to a mini window, you can drag it's volume slider without the window getting focused. Useful, therefore, to have just the volume slider visible in the corner of the screen.
Just noticed that the play controls work without iTunes getting focus too.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040302024635255