Currently

Some guy James wrote a package of json utilities for .net.  I don’t do .net but there’s still something really interesting in this package… a json schema language and validation implementation.  I can’t find specs for the schema language, but here’s the project page, docs for the json .net classes, and a demo of the validation engine.

You can look at the demo for some examples of schemas.  The only other json validation stuff I’ve seen so far has been either convert to XML and then validate, or just simple well formedness validation.