I ask myself this question every single time I board an airplane, but until now I had never bothered to research the answer. I guess I was just waiting for someone else to ask.

The Federal Aviation Administration explains:

The FAA is concerned that there are still many unknowns about the radio signals that portable electronic devices (PEDs) and cell phones give off. These signals, especially in large quantities and emitted over a long time, may unintentionally affect aircraft communications, navigation, flight control and electronic equipment.

Exactly how this interference is supposed to happen is unclear, but perhaps a pilot listening to information about coordinates would instead be briefly subjected to a teenager's argument with her boyfriend or (even worse) a few seconds of "My Humps." Or maybe the pilot just wouldn't hear anything. In certain situations — particularly during takeoffs and landings, when the plane is most vulnerable — this could have a serious impact. In these situations, pilots can't afford to be distracted or to lose communication with air traffic controllers, even for a second.

When the plane is safely in the air, there's more margin for error. A brief loss of communication or a slight change in trajectory while the plane is cruising is a minor thing — it can probably be ignored or corrected without any risk to the passengers. That's why most PEDs are permitted above 10,000 feet. Cell phones, however, are usually banned throughout the entire flight. This ban actually doesn't come from the FAA, but from the Federal Communications Commission (FCC). Why? Well, the FAA site says:

Cell phones (and other intentional transmitters) differ from most PEDs in that they send out signals strong enough to be received at distances far away from the user.

The FCC isn't motivated by safety in this case (no one wants plane crashes, of course), but rather by the desire to avoid overloading their ground networks. You see, when you're up at 35,000 feet and you turn on your cell phone, the additional height can enable your registration signal to go out to hundreds of towers at the same time (instead of one or two), creating unwanted network congestion. (The FCC is actually considering lifting this ban and using on-board equipment to limit signals.)

So that's the official answer, but it doesn't seem like a very good one to me. Skeptics (even some scientists) say this whole "interference" thing is hogwash, that the signals from our puny personal devices couldn't possibly have a noticeable impact on airplanes' million-dollar electronic suites. (If a plane's navigation system was really that fragile, air warfare might be decided by Walkmans instead of missiles.) Some have even gone so far as to suggest that airlines ban cell phones only because they want to force their passengers to use the ultra-expensive in-seat airphones.

Research has indeed shown that portable electronic devices aren't really much of a problem — a 1996 study failed to discover a single instance of cell phone interference. Obviously thousands of people have clandestinely turned on, or even talked on their cell phones in flight (even during takeoff and landing), with no ill effects. No plane crash has yet been attributed to interference from portable electronics.

I guess the airlines just prefer to take a "better safe than sorry" approach. If there's even a possibility that electronic devices can interfere with airplane equipment (and no one has yet proved that they can't), and if this interference could possibly contribute to a plane crash... well, asking passengers to temporarily turn off their iPods seems like a reasonable price to pay to prevent this from happening. At the same time, if you absolutely need to turn on your cell phone or BlackBerry for a few minutes to retrieve some piece of information, you shouldn't be too worried about the implications. It's not like the plane will crash the instant you turn on your phone.

Of course, I'd strongly recommend obeying all airline regulations, no matter how unnecessarily strict they seem. Even if there's a one-in-a-trillion chance that your PED will affect the plane's equipment, you don't want to be that one person who causes a problem.

Sources: FAA, FCC, Air Rage, USA Today