It probably heats up because the cell chip inside gets quite warm when it is sending or receiving data. If your battery life is short as well, you might want to check out the reasons why on the internet. They are probably related. Your iPhone uses the cellular chip for more than making calls.
Here's what I've found with my 3rd gen iPod Touch: when you're running apps that use a lot of battery, the device heats up. I've learned to constantly shut down the apps and not play for yours anymore.