Allow me to repeat myself somewhat and clarify my general opinion of JSON and its use in cross-domain browser scripting:
Happy Fun JSON is not really an API. Happy Fun JSON is not a bold declaration of side-taking in the grand war of web service specifications. Do not base business models on Happy Fun JSON. Caution: Happy Fun JSON may suddenly accelerate to dangerous speeds or stop altogether. Happy Fun JSON contains a liquid core, which, if exposed due to rupture, should not be touched, inhaled, or looked at. Ingredients of Happy Fun JSON include an unknown glowing substance which fell to Earth, presumably from outer space. If Happy Fun JSON begins to smoke, get away immediately. Seek shelter and cover head.
Do not taunt Happy Fun JSON.
Having said all that? Happy Fun JSON is pretty fun to throw around.