Not to mention someone crafty enough might be able to modify a phone for the specific purpose of interfering with equipment.
Yes, too bad that such a person would have to then keep this device turned off, because it's the rules. Curses, foiled again.
Hey, I just had a great idea. Instead of metal detectors, naked body scanners, and air marshals, why don't we just put up a sign on the cockpit door that says "No terrorism allowed on this aircraft". That would be a much easier and cheaper way of securing our airways!