Our little iPhone MMORPG has gotten suddenly exponentially bigger! We're having some trouble with bandwidth to the little iPhone clients [especially on 3G].

Now we just added support for "Player generated content" which also gets returned via PHP calls (JSON):
Missions and Player Generated Content

Any tips on compressing JSON returns from PHP -> iPhone?

Any libraries that do delta-compression of JSON returns?
[e.g. only send JSON data that has changed since the last call was made in the current session?]

Our database (amazon) can keep up... but our clients are having trouble fetching all the data in reasonable latency times. [but we know much of the data has not changed since the last game cycle].

Any help/libraries?