Ratings are a simple way for users to review a resource without much effort and is adopted by many online service such a BizRate and the Internet Movie Database (IMDB). These services are, however, closely tied in with the source repositories and every new rating service has to be custom-built.
Recently, the Open Archives Initiative developed a standard way to access the data stored in a single archive. If we use this technology to store ratings we can access them using a standard interface, hence providing a rating service to any OAI archive.
Taking this one step further, if the protocol is appropriately designed, then rating information can be exchanged among archives. Thus, hypothetically, if a person rates a movie on Amazon, that rating could filter into the IMDB.
There are some unanswered questions: how to handle different systems of ratings, how to prevent duplicate rating by the same person, whether or not to allow for users or entire archives to be weighted for importance, etc.
This is a problem that looms large for new federated systems like NSDL and a prototype system may shed much light on how to solve the problem.