Think about this next time a flight attendant tells you to turn off your phone. According to a confidential industry study, cellphones and other personal electronic devices really do pose a safety threat to airplanes. A report by the International Air Traffic Association documents 75 incidents of possible electronic interference linked to cellphones and other devices. And they're scary incidents, like the autopilot disengaging itself at 4,500 feet. Other incidents show interference with the navigation and communications systems.