I just got a new desktop machine and was faced with this problem: How to transfer the ratings when I move my music collection from the old machine to the new? I have over two years of metadata stored up on which songs I like, and I'd hate to lose it.
Here are a couple of ways, thanks to pointers from a dev and tester on the windows media team:
#1: Use this (unsupported) tool to back up and restore your ratings.
#2: Run the Windows Media Player 10 beta and turn on Tools | Options | Library | Maintain my star ratings as global ratings in the media files.
Both of these options add the ratings to the actual MP3 files.
I initially had some problems getting either of these to work for me, but I eventually succeeded. I'm not sure if my experiences were universal or not, so give them a try.