I think that acts_as_rateable seems to be quite obsolete and doesn't support AJAX.
I'm looking for a simple "5 star" rating system: is there any plugin or tutorial that can help?
Fully a year later, since this is one of the top hits on Google for this subject I thought I'd add this note:
Ajaxful rating doesn't really work on Rails 3, so you might want to check out this tutorial for how to create your own star rating system in Rails 3. (http://eighty-b.tumblr.com/post/1569674815/creating-an-ajaxified-star-rating-system-in-rails-3)