Bigwullie, which part of the world are we talking about?
Here in the UK it seems that the signal disappears between 1500 and 2000ft. (that is on the very few occasions I looked at it.)
Dixi188,
Yes, the safety brief is there for everybody's safety and you should abide it. But people do forget sometimes and you are not excluded from that. However when somebody leaves their mobile phone on during the flight it will have no impact what so ever on the aircraft.
Think about it. Several airlines a contemplating the idea of allowing passengers to use thier mobile phones during flight. The only thing required is a bit of extra equipment but no other changes to the already excisting aircraft systems. So in a year time you might be sitting in exactly the same aircraft as yesterday but now all 300 passengers may use thier mobile phone.