Oh and sorry for not specifically answering your question.
Yes, heat is perfectly normal and acceptable when charging your iPhone. You will notice more heat when charging from the wall in comparison with USB from a computer due to the difference in ampage
Charging a battery involves chemical changes caused by the current flow thru the battery - as it is not a 100% efficient process some heat is also produced.
It is a bit like using an incandescent light bulb - all the electricity is not turned into light, so the bulb gets hot.
For what it's worth, my iPhone 4 also gets 'really hot' when charging through the wall. Although this is not my first Apple product, it is my first iPhone and so it is difficult to gauge exactly how hot is too hot. In comparison, my iPod Touch 2G does not get this hot. Having a product whose outer surfaces are made almost exclusively of glass and steel does not make the assessment any easier. I have even received the temperature warning on two occasions while charging the phone.
It also seems that my iPhone gets very warm in use, especially if it is used for more than 10 minutes at a time. I understand that any mobile device is going to get warm when used, but the degree of warmth perceived is somewhat disconcerting, especially when I read comments that attest to very little change in temperature even when charging.
Mine seems to get less hot when the wall cord is plugged directly into an outlet. But if the wall cord is plugged into an extension cord or surge protector strip ot gets pretty dang warm when battery is super low.