My iPhone 4 gets hot as the battery is charged. To compound the issue, I think I bought a cheap case that traps heat, which in turn makes the phone even hotter -- you can feel the heat if you hold it in your hand while it is charging. When you stop charging it, the excess heat goes away.
My question is: does this damage the hardware? Also, to reduce the heat, I put the phone on the floor (which is cooler) and I found that the device was not as hot as before (previously I had put the phone to charge on a wooden table). I am unsure as to whether my measure helps in elongating the battery life and protecting the electronic components.