Just got back from a 3 hour trip using my new iPhone 5 to navigate using turn by turn (which worked really well). I had the iPhone plugged into a cigarette->usb adaptor. About 1.5 hours into the trip I noticed that the screen had turned off, after unlocking the iPhone, it popped up a message with the title "Temperature" and content along the lines of "the iPhone needs to cool down" and we are straight back to the screen off and locked. After this, I unplugged it and let it cool down, after which it was able to finish navigating the route on battery power. Has anyone else been having their iPhone 5 warn them about getting too hot and needing to cool off? It seems like charging the phone was a large part of this problem.