That would be really nice. My laptop has that for the screen/back lit keyboard and my laptop is a $1k of the shelf laptop. The model S has automatic windshield wipers, when rain hit the windshield they turn on (http://youtu.be/7TF0gGnBqF0?t=1m14s). An ambient light sensor is probably a very inexpensive part. I'd be surprised if it wasn't on there.