I have replicated this problem a few times today. I am running my iPhone4 with Navigon using the GPS and plugged into the charger. It gets quite warm. I noticed if it gets to warm the brightness drops down quite a bit. 100% Brightness looks more like 20%. Going into the brightness setting will not bring it back up to full level but it will change the brightness. When I turn it off and wait about 30 seconds and let it cool down and turn it back on again, it goes to full brightness and it is fine until it heats up again. While driving it was ok in the car because of the AC but I left it on during Rest Stops and Lunch breaks and that is when it overheated and the brightness went down.