I've just finished an Applescript that rates the songs of your whole iTunes library or of a specific playlist. It rates them based on how often each song is played in relation to every other song in the list.
Here's the source and the app. It's not very elegant -- there are no progress indicators or anything. But hey, it gets the job done. And, for me, it's finally given me a useful way to use iTunes' ratings. Have a look and see if it can figure out your favorite songs.
EDIT: I guess I accidentally saved the source code as run only. (CRAP!)