Over at TUAW they have the following:

Do you like giving ratings to songs in iTunes? If so, then you’ve probably noticed that you are only able to rate songs on a full-star basis, not enough granularity for some music fans… there’s a longstanding AppleScript hack to enable half-stars, but now there’s an easier way around this issue. Macworld’s Rob Griffiths found a work around, involving a simple Terminal tip to enable half-star ratings.

And how do they do this? Close iTunes, and ith the following command in Terminal:

defaults write com.apple.iTunes allow-half-stars -bool TRUE