Nah, I used my iPhone all through the flight to New York, and the return flight too. I just had it in "Airplane mode" incase anyone asked, and so long as it's stowed away (pocket, back etc) during take off or landing, you're fine. I was using mine for music mainly, but games, movies etc, aswell

Plenty of people use laptops on planes - if mine had a battery that lasted more than 3 seconds I'd have taken that and used that for watching movies on, as that'd be way better than in flight movies!!!