Babbage reveals a major reason why flight attendants have to nag you about it during takeoff and landing:
The problem is that the current [FAA] guidelines require each airline to test every make and model of each gizmo it wants the FAA to approve for use on its flights—and then to do the same for every type of aircraft in its fleet. The airlines have baulked at such a monumental task because of the cost. The FAA is now looking for ways to bring airlines, aircraft manufacturers, electronics makers and other interested parties together to streamline the certification process at least for tablets, e-readers, game machines and a few other popular gadgets.
But do not expect such easing to extend to phones. One reason is that, with the enormous number of makes and models in existence, getting all mobiles approved for use on board aircraft would be prohibitively expensive. Another is that the ground-based interference problem has still to be resolved.
The head of the FCC has even urged the FAA to allow more portable devices. Meanwhile, flight crews are exempt from the restrictions:
[F]light crews have had permission from the FAA to use portable computers called "electronic flight bags" in the cockpit since the early 1990s. Today, they carry iPads and other tablets as replacements for the bulky aircraft operating manuals, flight-crew manuals and navigation charts. These portable electronic devices are in much closer proximity to the aircraft’s avionics than anything passengers are likely to bring aboard, and remain switched on throughout the flight.