I think it's worth pointing out that both Firefox and Chrome support the non-standard JSON that Python supports (serializing and deserializing basic types).  I'm guessing that communicating with web browsers is the vast majority of JSON IPC.  That is to say, supporting the de-facto standard implemented by web browsers may be better than adhering to the exact specifications of the RFC.

Maybe someone else wants to check what IE, Safari, Opera, and the various phone browsers allow as that might influece the discussion one way or another.