Well, many recent phone easily become very hot!
Sometimes, playing GTA3 for a long time, my HTC Sensation goes to 50°C !
But it bener goes so hot with fpse...let's say 45°C
And it's not the battery that's so hot, it's rather the part of the phone where the CPU is
We're running high frequencies, dual cores, without any fan to cool the system^^
I mean, have you tried other really heavy android 3D games? Or just watching an HD (720 or 1080p) movie on your device? I'm pretty sure it will become as hot as when playing Fpse.
If your device is new, and if you're comming from a less powerful device (single core, 1ghz max), you may be surprised by the way a recent device can 'normally" become hot
Usually there's a protection against overheating, your device is supposed to reduce its frequency and/or simply shut down in real extreme case