iTunes 7 now keeps track of the frequency (Skip Count) and last date of skipping (Last Skipped). Two more useful smart playlist criteria to weed out seldom listened-to tracks.
[robg adds: Control-click on the column headers to add either of these to the displayed columns...I'm particularly pleased with the skip count feature, and have already put it to use in a couple of smart playlists.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060913130054623