It's my understanding from the Netflix TOS for their API that ratings can't be stored by a third party, for privacy reasons. Perhaps that has changed, or perhaps I read it wrong. I was skimming out of curiosity.
A couple of relevant threads-
http://developer.netflix.com/forum/read/49287
http://developer.netflix.com/forum/read/52540
In short, it looks like you can't get full ratings history, though you can get up to 500 of them. And you can't store for more than 24 hours.
Though several months ago Netflix said they were looking to change the api so you could get the full history and also to change their TOS so you could be allowed to store it as a third party (with sufficient notification to the user that you were doing so).
So it may work out in the not too distant future.