I've noticed my iPhone is getting a little hotter than I thinks should be acceptable. I imagine some heat is normal, but I'm getting a little worried. Its worst when I'm using my belkin car charger and a FireWire wall plug, but it also happens with the USB plug that came with the phone, just not as severe. Its also getting fairly hot unplugged when I've been using it for a prolonged period of time. I imagine this might be normal? How hot is too hot?