Mobile phones nowadays cause less EMF than those of yesteryear, but unless the aircraft has aeromobile i dont see the point in having them on as you wont get very good reception at 36000 feet!
In case we need to evacuate an aircraft, we have to do it in less than 90 seconds!!! Thats not a lot of time at all. In the case of a 2 class 777-300 it means getting 4.5 pax off the plane per second... If everybody were allowed to use their ipods, laptops, cellphones, blackberries and whatnots, imagine how many things would be lying around the cabin, causing people to fall over, and in turn block aisles and exits... Every second counts!
If we start letting pax get away with using their PED's, soon they will ignore that instruction all together... So if you feel up to it, tell them directly, or contact the crew as soon as practical... i.e after takeoff!
I was deadheading once to Tunis, and we passed through some heavy thunderstorms on approach. The captain told the crew to sit down immediately over the PA. I noticed a pax across the aisle not wearing his seatbelt... I politely suggested that he should put it on, as it was probably going to get very bumpy very soon! He laughed at me and said "Im a frequent flyer..." I had done my part! About a minute later, the plane goes into a nosedive lasting a few seconds! The guy was floating in the air! Then the plane suddenly pulled up again, and he landed with a loud bang on the armrest! I have never heard a man scream so loud! It turns out he had broken his tailbone on the armrest! So much for being a frequent flyer!!!