Not to state the obvious but we all tend to forget that these things are not phones they are a combination of computer, high frequency radios, storage arrays and high def monitor screens, they will act and react like computers and radios and screens, all of which get warm when they are working.
My phone also gets warm, sometimes very warm when I am doing heavy processing, but that is not the same as HOT. The phone has a dual core and so when streaming or using applications that put heavy processing demands on the chip it will get warm. If you have a case this can keep some of the heat in as well, the verizon vinyl cases are especially good at this.