The safety risk is only there if everyone on the flight switched their mobiles on at the same time. This sudden surge in power might possibly upset something but its a very big might. Normal use is not a flight risk. I have tested all of the sensitive systems including the HIRF checks on certain wiring looms and never turn my mobile off and have never had any trouble at all. It's only the increase in power trying to connect to a base station that multiplied many time might give a problem. It is probably safer to leave things on rather than have them all switched on at the same time.
