Sort of related. Riding home from work last week, with walkman blaring away. (I know! but it was on a canal towpath). I shoved the player, my keys and a spare (rechargeable) aa battery in my shorts pockets, as I usually do. I was suddenly aware of a hot feeling in my pants (!) and discovered that said battery was pretty warm. Wasn't warm enough to burn skin, but pretty uncomfortable all the same.
Got me wondering about what I'll put in my suitcase next time I fly commercially.