Cell Phones, Mythbusters, and Selective Perception
The situation became clearer when I found a fairly detailed webbed description of the program. The people at Mythbusters were unable to get permission to actually test a cell phone in an airplane in the air--cell phones being forbidden to operate in airplanes in the air. Instead, they did two experiments:
1: A signal generator to simulate a cell phone, a navigation gauge, and various other equipment were placed inside a faraday cage (to eliminate external interference). They found that signals in the 800-900 MHz range affected the navigation gauge.
2. An actual airplane was borrowed and tested on the ground. No signal they tested had any effect on its electronic equipment. They concluded that the difference was that the electronic equipment in a real airplane is shielded, whereas in the first experiment the signal generator was more or less next to the unshielded navigation gauge.
So it is true both that they found that a cell phone had no effect on an airplane's electronics and that they found that some cell phone signals could affect the kind of electronic equipment in an airplane. I suspect that listeners selectively remembered the part of that that fit their existing beliefs and prejudices.
I also found a discussion of the issue of cell phones on airplanes, from the sceptical side of the argument, in a Computerworld article. The author thinks that both the regulators and the airlines are in favor of the ban for reasons unrelated to safety risks from interference. The article provided a link to an FCC press release which amounts to "we decided to stop our investigation into whether cell phones should be banned on airplanes because we couldn't find enough evidence to tell," which strikes me as some evidence in favor of the sceptics' view of the situation.