I'm not sure if this was possible under iTunes 3-, but as I recall, it was complained about. It appears that under iTunes 4, you can very easily reset the rating for a single song, without having to jump through hoops to do so.
Simply select the song in your library and click just to the left of the first star. It should now have no stars set. You have to click a little closer to the grid line than to the first star.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030430075943631