I'm using musikcube as player for my music and it has 5 star ratings. So does windows media player on my pocketpc. So does iTunes. But they never translate between them!
How could they?
One way would be to store it in the id3 comments tag. Both id31 and id3v2 formats support comments. This is a great place to store ratings as text. ***** is 5 stars and anyone can read it. What would make sense is to store ratings in the last 5 characters of the comment field. Then, when changing or loading ratings, you can push or pull from that location.
After doing a little more research, I found that id3v2 supports freeform key=value tagging. Now it seems that all you need is for players to start reading and writing a custom ratings tag.
Why is this important? Because time you spend rating songs in itunes or musikcube or anything else is valuable. That's valuable info about your song that only you know. If it is lost everytime you switch from an ipod to an iriver, it is less valuable.
If I download a creative commons track or rip a cd at work and rate it in musikcube, I should be able to take that track to my home and have my ratings show up there. The information I put into it loses value if it is bound up in one location or one player.