We have two Droids in our house and they both get fairly warm when talking on calls. At first I thought it was just because I was holding it near my ear. But then realized it was a lot warmer than it should be normally. We live in a spot where our signal is usually weak. So what Hazydave says about the cell tower telling your phone to boost the signal makes sense. However, I have never had a cell phone warm up like this; and they have all received weak signals. Usually, the battery runs down quicker trying to stay in touch with a tower.
So I wonder if anyone else's Droid heats up because of a weak signal and also while talking. Almost every one else on this thread says their's heats up when using the browser.