Overheating problem and reduced charge
When this has happened on my 3GS I would reboot it just in case.

I've never determined the cause, but it does appear to heavily use the cellular radio and the CPU to maximum power, both of which generate significant heat and reduce the battery charge significantly.

Since it happened so rarely - perhaps a few times a year - I assumed it was some weird iOS glitch and I chose not to pursue it any further.

If it's happening to you more than a few times a year, though, I'd take it in and have it replaced.