I have been noticing that many people do not turn their cell phones off during flights, like the airline asks them to do. The reason they ask you to do this is because cell phones transmit signals, even when not in use, that can interfere with navigation systems.
A recent IEEE study/experiment found that someone makes a phone call during takeoff and/or landing approximately once in eight flights. What I think probably happened was that someone was sending e-mails via their blackberry, trio or whatever. The experiment was pretty informal, but clearly highlights a problem.
I don’t worry about the takeoff interference so much as I worry about 100% ILS landings, where the pilot is completely reliant on the instruments.
Unfortunately, the FAA has taken a weak position on this issue, and asked that the airlines police themselves. Additionaly, due to customer demand, the airlines are putting pressure on the FAA to allow use of cell phones on flights. There has been little research in this area, and the FAA expects to get the research data from the airlines. Keep in mind that these are the same airlines that are operating under bankruptcy protection and can not afford to pay their employees what they have promised to them.
I really expect this issue will be decided due to a catastrophic event. I hope that I am wrong.