I had always heard that the real reason they make you turn off electronic devices is so that you listen fully to any instructions you are given. Why else would they make me turn off my wi-fi only Kindle?
Maybe. I think that the "cellphone interference" is just a blanket term they use whenever anything goes wrong. When I was flying from PSP to DFW a few months ago, the flight attendants had already given the "turn off all electronic devices" thing followed by the safety brief, yet we still hadn't moved onto the runway. Instead of telling us what the hold up was, the flight attendant got on the intercom and said, "We would be on the runway right now, but somebody left their cell phone on and it's interfering with our signals." Lo and behold, about half the passengers pulled out their cell phones and turned them off. This was reverse psychology, shifting the blame to the passengers for the delay. Sad thing is, it worked.