A platform for this has existed almost as long as the web itself. PICS is a W3C standard for labeling content. There is a working draft out for an RDF-based version of PICS called POWDER.
PICS ratings are assigned by the IRCA, now part of The Family Online Safety Institute.
The problems with PICS adoption were predictable. First, like with the V-Chip, few parents enabled content filtering on their browsers, or even knew that feature was available. Second, the honest content providers, the ones who got their sites labeled for language, violence, or adult situations, got screwed because of all the sites out there who didn't opt into the system, and which could be viewed by everyone.
The possession of a book becomes a substitute for reading it. -- Anthony Burgess