In iTunes 6.0.2 on a Mac, when you are configuring a Smart Playlist rule that includes My Rating, the stars you select to determine the ratings for the rule may not work as expected. Apple explains what you need to do to get the results you want.
Basically, you have to bracket the star rating (say three stars) with a 'match all' rule for 'rating is greater than n-1 (two) stars' and 'rating is less than n+1 (four) stars'.
[robg adds: I assume this bug will be squashed in iTunes 6.0.3, but in the interim, here's the workaround.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060123184519994