The way it was explained to me was thus:-
When a mobile phone is switched on it sends out a signal to the nearest base station, a sort of "here I am" message. The further the phone moves from the base station the more power the phone uses to try and reach it, until it gets to full power. Now one mobile phone on a plane would probably not effect any of the planes systems, but envisage 300 mobiles all going up to full power together and things might be a bit different. The answer of course is to build a base station into the aircraft then all the mobiles can operate at minimum power and I think this technology will be coming soon.
The other reason of course is that the cabin staff want your undivided attention for the safety announcements.