well you never see a car electronics go haywire when your phone goes off right? and theres a lot more cars than planes so it would be more likely if a phone could interfere with an electronic system it would happen in a vehicle.
as for the number, it was pulled out of my ass. But that has to be the only reason why we have to turn off all electronics (phones, mp3 players, portable game systems and dvd players) Just on the freak chance that many of them will send off a signal on the same frequency at the same time.