Wanna know who is listening to your iTunes library? Use the Rendezwho widget for Konfabulator.
I have some issues with it, but I know of no other application that does what it does. Unfortunately it can't tell you anything other than the IP address of someone who is leeching off of you (someone who is listening but not sharing themselves). If they are sharing, it tells you the name of their shared library which sometimes is quite useful (especially when their library is called "Jone Doe's Music Library" but probably a lot less useful when it's called "Cool Tunes").
Now you know who's connected. But what if you want to know what song someone is listening to? Well, it's listed in the Activity Monitor application (usually found in the Utilities sub-folder). Click on the iTunes listing in the Activity Monitor window and then hit the Inspect icon (the one with the italic letter "i"). Click on the Open Files tab, and at the bottom of the list is the IP addresses of the people who are connected to you. And if someone is playing a song, then it lists it under there. Remember, that just because someone is connected to your library doesn't mean they are actually playing a song.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040803175335892