This is really easy to do and a fun experiment to learn MITM attacks. My boss was gloating he was the best at jetpack joyride and the whole office had a good laugh knowing that I could always beat his best score by a small amount the day after het set it. Apparently that put a downer on his christmas because he spent so much time trying to beat me.

It did cross my mind whilst watching the traffic if you could spoof in app purchases but never took it any further.