The Netflix challenge forums are a treasure trove when it comes to this particular ML problem.
In the end the algorithms grew into huge monsters blending results from tens of algorithms, but the most interesting discoveries were made in the first year IMO.
Wow, I never thought of looking there. It's amazing reading tricks like that which people have come up with after years of thinking and experimenting. And they seem usable for quite a wide range of rating related problems.
http://www.netflixprize.com/community/