You're not going to get it hot enough to do any damage. While I don't know (remember) average heat, electronics will start to fail at... typically, is the design and function of what it is, that will determine safe operating temperatures.
Cell phones, by nature run hot. Those batteries, get hot. The CPU/GPU run hot. The phone is designed for heat, far in excess of 105-106. Well, beyond the point of you being able to hold onto it any more.
The Captivate is notorious for running hot. Man, you talk on the phone for 15 minutes and that ****** felt like a hot potato. There were times, that I had to end phone calls, because it was uncomfortable holding it. My wife inherited it after I upgraded. Still runs like a champ (besides some Serendipity bugs), looks like one and my wife loves it.
Sorry, for no concrete answer, but honestly... the phone would probably start to melt in your hand (plastic would turn soft and pliable), your screen would also start to react, with distorted images/colors.