This is a really nice library, something that I would love to see in the standard. The client code I wrote was really simple and clean. You really start to see the goodness of C++11 when you use a library like this.
One comment - is there a reason why the field/key operators in the json::value class only return by reference and not by const reference as well?
Something like this:
void kung_foo(json::value const& jobject)
int id = static_cast<int>(jobject["id"].as_double());