Actually the other day, I noticed my iPhone in my pocet at work was ridiculously hot against my leg.
I pulled it out and the screen was off, everything was normal.
I restarted the iPhone, and the heat quickly cooled. I'm guessing one of the apps was still running despite iPhone being on the home screen and in standby, because the battery had lost all its charge in the 4 hours i'd been at work.
One recharge later, and all was fine again. Its not happenned since.