The phone uses the back metal case as a way to dissipate extra heat. It should not get so hot that you can't touch it though. If your iphone (or wall charger) is burning you then return them. Here is what Aple has to say about it.
http://docs.info.apple.com/article.html?artnum=305695
- SR
Hmm, anyone useing a skin have a problem with a hot iPhone?