From my understanding JSONP can only be achieved using the GET verb. Assuming this is true which I think it is, then this rules out core compliance with true REST in which you should make use of different verbs i.e. GET,PUT,POST,DELETE etc... for different and specific purposes.

My question is what type of barriers am I likely to come up against if I where to say allow updating and deleting of resources using a JSONP service using a get request.

Is it better practice to offer a JSON service and state that the user will need a server side proxy to consume using JavaScript XDomain?

Cheers ,

Andrew