A bit of shameless self-promotion here, but I thought the community might be interested.
I made two Quicksilver scripts that plug into iTunes: one to add tags to songs in the 'Comments' field of an MP3's ID3 tag, and another to quickly and dynamically generate playlists based on tagged songs. Think del.icio.us for iTunes.
I've found these scripts have greatly aided my music listening experience. This is freeware, under the Creative Commons license. The scripts and bit more detail about the project can be found in this post on my website.
[robg adds: Since I'm a Butler user, I contacted Peter Maurer and asked if he could "genericize" these scripts. He could and he did; you can download the generic scripts right here (12KB). I had success using these in Butler as AppleScript actions assigned to a hot key. They should also work, perhaps with some modification, with LaunchBar.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2006011710273170