Everyone always talks about how it feels hot and metal is different than plastic. How about real numbers from something like the Battery Monitor Widget? Mine regularly hits 120F according to BMW when playing Ingress with phone in car dock. I even hit 132F once today (I am sure this is not good for chips or battery).
For reference regular charging at my desk or at home at 900+mA never gets above 92-94F.
I think if you are trying to charge at the same time it shuts off charging around 115F and the LED starts to flash red. I am sure that is too high, but I have been to Sprint stores twice telling them that this is too high, but they keep either saying it is Ingress or it is OK or that you should never run GPS and charge at the same time. I have to run a test with Maps for navigation again, but I think the same thing happens there. Having GPS on, screen on and data just burns the thing up.