There are two issues going on simultaneously.

I suggest the following experiment. Take your hand-held two-way radio transmitter to Dayton Ohio during the annual ham convention there. Tune it to a 2 meter frequency, and just listen for half an hour. You will hear vast quantities of intermodulation--signals that aren't really there, and signals that are there that are hopelessly garbled.

This is happening because there are hundreds of similar transmitters all over the hamfest that are transmitting simultaneously. The intermodulation distortion comes about because when a radio receiver, half of your two-way radio, receives multiple signals these signals combine in the receiver and produce distortion signals that are the sum and difference of two frequencies. So with many multiples of frequencies that are rather strong, there are a host of fake signals going to your earphone.

Each device, such as an iPad, iPhone, android device, has multiple radio transmitters. Some of them are deliberate radio transmitters, such as the wi-fi or the cellular radio. Others are the side effect of the receivers that are built in to the devices, such as the GPS. Receivers themselves actually transmit, albeit at a smaller level. Similarly, if you have a transistor am/fm radio receiver, it generates (transmits) a signal as well.

Other unintentional transmitters are the clock for any CPU that might be there.

So on an airplane, if there are tens or hundreds of devices, you get a real mass of interfering signals.

How does this affect the avionics of the airplane? Well, you won't hear a heterodyne, as there is no audio output--just some instrument reading. What happens when the avionics hears intermodulation distortion?

As to the idea that two hundred ipads are just like one, the claim that “Electromagnetic energy doesn’t add up like that,” said Kevin Bothmann, the EMT Labs testing manager doesn't square with simple physics. Another article quoting this lab study suggested that he was confused by the difference between decibels and power levels. I argue that it adds up worse. Check out Dayton.

The article says None of those episodes have produced scientific evidence that a device can harm a plane’s operation. Reports of such interference have been purely speculation by pilots about the cause of a problem. The thing is, that speculation about problems in the air by experienced aircraft operators is the only clue that we have just up to the point where people die. As was the case in the Tenerife air disaster, three second long whistling sound (or heterodyne) blocked a very key bit of information.

The way we learn if a pilot's suspicion is true is for people to die in aircraft accidents.

So the second problem is for the cellular phones. Keep in mind that cellular phones depend upon line-of-sight. If you are walking down Jackson avenue to meet a friend at Intellegensia for coffee, your line of sight to the cellular towers is limited. So how far is the line of sight away if you are at 20,000 feet? Quite a ways. Possibly the next state. For every cell tower that your phone can "see", there is the possibility of a connection. Not a good idea to tie up that many cellular tower channels.

It will be interesting to see how this battle plays out. If the current FAA rules are right that passengers should not be transmitting during take off and landing and the social pressure causes the rules to change, the only way we will know is when a plane falls out of the sky, since we aren't giving pilots complaining about heterodynes any credibility.