This is a slightly circuitous way of finding this... If you have the Character palette activated (see previous hint) and you select the Stars/Asterisks category, you'll notice the very first star looks an awful lot like the iTunes3 ratings stars.
Place these stars in an MP3 file's comments box (by clicking Insert in the Character Palette with the comments box active), sort the Finder view by comment, and you now have a way to store and view ratings in the Finder, great if you delete/re-add mp3s constantly to your library, as iTunes doesn't store ratings info in the actual MP3.
Now we just need an Applescript to automate this both ways.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2002090406444829