Perhaps a light sensor? You are not going to use your mirror in a dark room. Unless you want the display to show up in the dark, but personally I wouldn't want that.

Motion sensing seems like a better solution to me. I can imagine looking from diagonally across the room wanting a quick glance at the current temp outside and it not showing up because it doesn't see my face.

Solid point. Maybe just a cheap wide angle lens from ebay and use easy off the shelf background subtraction from opencv at a low frequency as an easy of motion detection. I'm trying to think of ways to avoid integrating external peripherals that aren't part of the phone.