I think this will answer some of the queries regarding potential electronic interference etc which is the prime reason for the requirement. A much better source of information than me trying to go through the whole thing and screwing up all the detail.
http://en.wikipedia.org/wiki/Mobile_phones_on_aircraft
Just fyi there was an airborne test recently of a hand held device that detects any RF emission as the holder (FA or Air Marshal) walks up and down the aisle. Communications devices (cell phones, Wi Fi,and the like) generate specific types of signals when their receive circuits are in use (not in flight mode etc) These devices are already being deployed so those who might want to keep those phones on might also want to think twice before doing so.
Bottom line is still the same though...we ask you to turn them off..its not optional...so if you dont want to do it (as I said previously) dont fly...its that simple.
Cheers
MS