This might well be a very dumb question, but here goes anyway, I'm planning my turn-based ios game center game, obviously I know I'll need 2 iDevices to test it on, but how would I test the online turn-based play between the devices? As I'm to understand it, if the 2 devices are local (i'e close to each other) they automatically revert to a local communication method? is it possible to force them to communicate online so I can test that part of the project?
With GKTurnBasedMatch all communication is done between the device and the game center serves only, there's never any direct communication between two devices.
You don't actually need two devices, you only need two different game center accounts. (Although testing with a single device would be a pain.)