An excellent explanation of why FAR 91.21 prohibits the use of any device that intentionally radiates electromagnetic energy (e.g. moblile telephones etc.) in aircraft, at all times, unless testing has been conducted to ascertain its safe use. JARs include a similar prohibition. Note that for anyone wishing to use such a device, the onus is on them to prove that it is safe to do so, not the other way round.
So far, all available test results indicate that transmitting RF energy inside a metal tube, causes localised concentrations of energy that have adverse effects on other electronic equipment located within the same metal tube. Despite these findings, we will no doubt continue to be plagued by smart alecs who know better than the research scientists - as a group, mobile phone users aren't noted for their interest in anyone else's welfare.
The only answer to those who claim that the ban is senseless and insist on using their phones anyway, is to enforce the regulations and prosecute those who refuse to respect the safety of others. There are plenty of Aircraft Commanders lurking around here, lets hope they will do their duty in future.
**************************
Through difficulties to the cinema