A good example of this is a Comedian over here who successfully fought his charge for "using a mobile phone whilst driving" (a ridiculously specific law of dubious value) by arguing that he was using his mobile as a dictaphone and thus the law did not apply.
While I do not have any beef with the idea of government making regulations, policing cell phone usage in planes (or busses or trains or inside mcDonald restaurant) is not what I expect them to do.
I feel like this is going to be kind of a mess. "I'm not making a phone call, I'm voice-only skyping over wifi!"