[rgrieselhuber]

Doesn't require some third-party (which tend to lag behind the latest versions) library to get it working and I can write normal server side code in Python, etc.

[jauco]

You can run a REST server and make your callbacks using JSON. That's what I did a while back. PHP could be used without problems.

[mmastrac]

I've done the same in the past (Python backend). I'll add one more suggestion on top of that. You can create JSNI bindings for all of the JSON RPC objects with GWT to get compile-time API checks that disappear entirely from the output.