I understand that we're talking about whether or not cellphones cause electromagnetic interference, but I'm surprised that nobody has mentioned that the real reason cellphones, iPods, Walkmans (if any still exist), video games and the like be turned off is not that anybody cares whether they interfere with anything, but because the cabin crew wants the SLF to be at least minimally paying attention and not have their ears blocked with buds during the phase of flight when it just might be most important to hear instsructions from F/As--takeoff to 10,000 feet. Above that, you can immerse yourself in the moronic pursuit of your choice.
It's why cabin crews would equally forbid two tin cans on a taut string, if there are any ex-Boy Scouts out there who remember how that worked...