actually coming from a dork…is it the white 3GS version…if so it does in fact over heat and over time will discolor the back casing. I’m sure apple has known about this and will probably do something about it. But many other people are having this problem too. You aren’t the only one :)
As Ninetails stated previously, the white 3GS phones were reportedly overheating from some battery malfunction and were being replaced if you went to an apple store and inquired about it(also over the phone). But yes, they do seem to get hot. Its just if their scorching hot you may want to turn it off and put in front of a fan for a while.