I remember reading about these for the first time in MIT Technology Review when I was in high school in the late 90s. And e-ink screens. I think that I always thought I'd see these first. Funny how that works out.
"Everyone who’s held a smartphone to the ear or watched a movie with a laptop balanced on their knees knows the devices get hot. Most are unaware the same battery technology is widely used in electric cars and has made few notable advances in a decade."
This is the first paragraph. Sounds sensible, but when you think about it, more heat from these devices is coming from the CPU, isn't it?
In the case of phones maybe it's the fact that you've just been holding it against your body for a few minutes.