In reply to wintertree:
I think you might have a good point. A bit like how they never tell you that they're dimming the cabin lights at night so that your eyes are adjusted and you can see if we crash.
It was never based much on reality anyway and has now been proven to be mostly pointless.
http://www.wired.co.uk/news/archive/2013-10/03/wifi-is-safe-on-planes
For a while now BA have let you use your mobile the second you touch down rather than after you've taxied to the gate.
Let's face it, if an electronic device left inadvertently on (which in a flight of a hundred passengers or more, there on average are bound to be a few) was so dangerous, surely they would make you check them at the door and turn them off.
I never fully switch off my laptop - just keep it in sleep mode which isn't really strictly according to the rules. I've left my mobile on by mistake before and had an old Android phone with an annoying button that I managed to turn on in mid-air in my pocket several times without it causing any problems.