I was asked to turn my iPhone off eben though it was in flight mode on approach into Cardiff from Larnaca with Thomson Airways in May.
I am pretty sure that Flybe recently have asked passengers to send photo's in of their journey with them and even published some as passengers walked around the aircraft wing as they disembarked the aircraft.
I suppose as long as your paying attention to the safety demonstration then there shouldn't really be much of an issues.
The worst than can happen is that they tell you to stop and as long as your not pointing it at the crew, other people on the flight, children etc then a few aerial shots can't really do any harm. If they do ask you to stop then obviously respect that and do stop.
Sometimes it can literally be down to the crew members on the day I suppose.