I suspect that the really simple answer is that nobody wants to find out for sure because it
MIGHT end the airline monopoly on telephone call charges. I think that, if anyone had ever wanted to run a properly constructed scientific trial on any PED (Portable Electronic Device), the first problem they'd encounter outside of the airline itself, is the myriad frequencies and potential interactions between the various types of PED.
Now, as one who designs instrument procedures, I know that the tolerances can (on occasion
) be right on the limit. Think about that for your next take-off and landing - then ask yourself... do I
really want to jeopardise any of those OCCASIONALLY tight tolerances?
On a personal note, I don't think that anyone really knows the answer to this so it's undoubtedly safer to err on the side of caution. I'm not yet convinced, one way or the other on the issue of PEDs, but I sure don't want to find out the HARD way.