After some poking around, I figured out that I'd read about a hidden iTunes preference that enables half-star ratings. After reading about it, I must have tested it, because it was enabled in my version of iTunes (confirmed via a defaults read... check in Terminal). Here's how to enable it on your copy of iTunes. First, quit iTunes, then enter this command in Terminal:
defaults write com.apple.iTunes allow-half-stars -bool TRUE
Relaunch iTunes, and you'll find you can now click between the stars (in the Ratings column) to get half-ratings. If you prefer to drag, you can do that, too, and you'll see the half-star values as you drag across the stars. Very cool; no more resorting to AppleScript tricks to get my half-stars!
[robg adds: This also works in the Rating box on the Options tab of a song's Get Info window. ]

