Chad Miller says, "your camera is stupid" in this Flickr Technique thread because it wants your image to be middle grey, like the square pictured here. Your camera can't know whether you're aiming it at snow or cinders, so it splits the difference.
Steve Traudt has The Fifty-Cent Solution for Apogee Photo Magazine.
David uses the back of his hand in a very similar way in this installment of On Assignment.
By Mike H.