I discovered by accident that there are alot of seemingly random keyboard shortcuts in itunes while the active selection is anything in the sidebar. What the keys do depends on what you have in your itunes sidebar in the way of named playlists, audiobooks, TV shows, etc.
First of all, named playlists take priority over everything else. For example if you have a playlist named Alternative Rock, then when you press the [A] key, iTunes will highlight that playlist. If you have no playlist that starts with an A, then it will take you to Audiobooks ... unless you have no Audiobooks, then it will take you to the "iTunes Store". See what I mean by seemingly random.
I have put together a list of shortcuts and the items they activate below (assuming that you have no playlists that start with that letter):
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060915095423700