I almost failed physics in college (one of the reasons I am an attorney ["I thought he was pre-med." "Pre-med, pre-law, what's the difference?" ]), but I vaguely recall a formula that the intensity of a magnetic or electrical field decreases in an inverse geometric factor to the distance away from the source. IIRC, as the distance doubles, the intensity is only one-quarter, etc. My math is horrible, but I think this is the relationship. Therefore, if a cell phone is right next to navigational equipment, the cell phone would be more likely to cause interference than if the cell phone were in the passenger cabin of an airliner. Also, as a previous poster noted, Boeing and Airbus (and presumably Bombardier and Embraer) design planes that are resistant to falling out of this sky. This presumably includes some insulation and shielding of critical electronic equipment, to guard against electrical interference. Just imagine lightning and its potential effect on aviation electronics.