I just posted to rubyforge (http://acts-as-rated.rubyforge.org) a new version of acts_as_Rated plugin. This version was modified to work correctly with MySQL, and added a new method rated_by?(rater) that returns true if the object was rated by the passed rater.
Thanks for this version go to Tiago Serafim, who tested om MySQL and proposed the rated_by? method with an implementation.