I've heard of these self heating foodstuffs like instant noodles and cans of tea but have still yet to see one I think. Wormgear has some details of one he played with the other day.
Original text from wormgear below.
Yesterday I was riding the Tohoku Shinkansen from Sendai to Tokyo. I got hungry so I decided to have ekiben (Eki Bento - Eki = Station, Bento = lunchbox). This particular ekiben was totally incredible! Inside the box were tender and delicious slices of beef tongue on top of a bowl of fluffy rice with some pickles on the side. It was very good! What made this bento so amazing, though, is the fact that it heated itself up!!! As you can see from the instructions, you just pull the string from the bottom of the box and it starts working. Within about 3 seconds, there was steam shooting out the corners of the box-- it got so hot that a little bit of the price tag started to turn black (you can see that in the lower corner there). After letting the bento sit for about 5 minutes, there's a nice, hot lunch ready to eat!! I have never seen anything like it-- it's amazing.
Check out my blog to see more! There is a close-up of the instructions there.