The latest episode of Mythbusters tackles the myth of cell phones on airplanes. Their experiment was done two different ways.
First they purchased a bunch of equipment, radio stacks, VOR/NDB radios, GPSs and got it working. They then tested the equipment by tuning it to an airport VOR (San Francisco, which was 10miles away).
During the entire experiment, they were trying to determine if a cell phone can cause interference with the VOR equipment. Either by deflecting the needle, or flipping the to/from flag.
Take a look at how VORs work at Wikipedia!
Through their experiments, they were able to make significant changes in the VOR by using 800-900MHz cell phones (old analogue and CDMA/TDMA phones). GSM, which works at 1800MHz didn’t affect any of the instruments.
Their second test was to try to duplicate the results in an actual working aircraft. They were able to do the experiment in a modern jet with a glass cockpit. They weren’t able to reproduce the same results with the old equipment they got from a scrap yard. The reason? The modern aircraft’s wiring is shielded against interference.
The Mythbusters concluded that the reason why cell phone usage is not allowed on airplanes is due to the fact that it would cost too much money to test every phone on every plane for interference. Better safe than sorry.