I like what AA do regarding mobiles. They say you can continue to use the phones on the round until they make an announcement, and then on landing they say we'll announce when you can use them again. In both cases they try to give you as much time as possible then a clear cut-off, which is a great way of doing it.

On take-off the cut-off is as the aircraft is reaching the runway, and on landing it's allowed once you're off the runway. Larger electronic items (laptops etc) still have to be stowed early as normal. With BA they say no mobiles from doors closed on take-off, and on landing until you've reached the gate, which is stricter and hence probably invites more chancers who think they know better.