The Beech King Air, a very popular short haul corporate turboprop has a warning regarding the Instrument Landing System. If you run the prop speed at 1800 rpm on an ILS approach the information provided by the ILS system won't be accurate.
Another instance I know of is a multi band radio used quite commonly in the helicopter industry that if you broadcast in one of the available bands also can effect ILS signals.
Those are both known issues that have been tested, confirmed and mitigated through warnings made available to aircrew. None of the electronic gear in the back of a commercial airliner have been tested for electronic interference. Even if the FAA did test all the gear out there today, there would be 100 new devices tomorrow, and the day after that etc.
I would also add that despite the small percentage chance of a mishap in commercial aviation, takeoffs and landings are still critical phases of flight . The risk is higher and the staff on those airplanes would like to have immediate capability to give instructions. Imagine being on the US Air flight that landed in the Hudson, trying to give emergency instructions if half the flight was grooving on their iPods.
I stopped turning off my cellphone and reader years ago. I promise you the crews don't turn theirs off. How many times (if you fly) have you watched a stewardess texting after they take their seats for landing?