I'd be better if phones came with car crash detection instead. It's quite easy to do (subject moving a more than 20mph, sudden halt, huge haptic feedback on the sensor) and could save lives more than a panic button. Hell with Bluetooth enabled car, even air bag deployment could send a signal to a phone to enable GPS and call 911 automaticaly.
Wouldn't it be sufficient to put car-crash detection in cars themselves? And call it "On Star" or something?