Version:           1.94 (using 4.1.3 (KDE 4.1.3), Debian packages)
Compiler:          cc
OS:                Linux (i686) release 2.6.27-1-686

It looks like Amarok does not distinguish between "not rated" and "0 stars", at least there's nothing visible from inside the GUI.

I don't know if this can be done at the database level (by allowing NULL for not rated), I'm too lazy to look at the code and have no idea how to look inside a MySQL-embedded-DB. 

My pledge is to allow to set the rating explicitly to "not rated". This can be done by
 - either defining "0 stars" as "not rated" without the need of allowing NULL in the DB and without having to change the GUI (though giving some feedback when removing the rating by clicking 0 stars). In this case 0.5 would be the lowest rating.
 - or by introducing a visual difference between "0 stars" and "not rated". This needs NULL in the DB field and slightly bigger change to the rating GUI, but is certainly more elegant.


Example purpose:

Use the rating system to filter out poor tracks (rated, <2 stars) from an album, but do not filter the unrated tracks at the same time. Do this by querying "-rating:<2" in the collection search.


Note:

After playing around for a while it looks like there is a difference between "not rated" and "0 stars" but it is not clear how and whether the rating is removed or set to 0.