Abrahm is correct. Only a hand full of devices are allowed to be on at all times on a plane. That list includes things like pace makers, watches, hearing aids and pagers in receive mode only. Other items can be used at the gate, but once the plane is rolling for any reason all other items should be off as they can interfere with the aircraft radios. Once in the air above a certain altitude, things like cameras and such can be used, as long as any transmit functions have been turned off. This stuff is right out of the operations handbook at my airline.