A few years ago I had a Motorola mobile phone battery overheat and start smoking while in my trouser pocket. I was in WH Smiths at the time with an armful of shopping, and had to leave in a hurry while being chased by the store staff....
Anyway, got outside and the phone was almost too hot to touch, but I managed to get the battery out - and the discharge and overheating stopped.
Moral of the story? If the battery isn't "in place" in a device its unlikely to discharge and so overheat. Make passenger remove batteries from phones and computers before boarding
Incidentally - I returned the phone and battery to Motorola for checks - and they said the problem was due to "water damage" - which was absolute rubbish.
I don't care what anyone else may say - personal experience proves that lithium cells are a serious risk. I could've had me b******* burnt off!