I just noticed that the latest version of iTunes (6.0.2) now displays half-star ratings in the UI. Question: How do you set half-star ratings? Answer: With Synergy Advance. I’ve added an entry to the FAQ explaining how to do this.
Here’s a screen shot of the development version of Synergy Advance displaying half-star ratings using Growl notifications (this is version 0.3 which will be released shortly).