Ok, to be more clear, I am looking at a personal electronic device that would automatically switch itself into 'flightsafe mode' (for example switching off the GSM signal on a mobile phone) if the passenger forgot to do so. Therefore it has to be clever enough to realise the aircraft is taking off, and then switch back to normal mode once it senses the deceleration of landing.
From what I've read so far, the vagaries of different aircraft and approaches would mean there would not be a foolproof way of doing this based on either pressure or acceleration.