A number of companies had LCD monitors planted in rear view mirrors to show off at SEMA. For example, Rostra Precision Controls had a display that could take input from three different sources, such as a rear view camera, a rear seat camera or babycam, and a navigation display. I didn't find the navigation display to be all that intuitive, since you want your maps closer to eye level, but the rear view camera makes a lot of sense. All factory rear view systems I've seen display in a center stack LCD, which isn't very intuitive since we're all trained to look at the rear view mirror. But the rear view mirror is the most obvious place to put the display that I'm surprised automakers didn't think of it first.
And rear view systems are likely to become much more prevalent. Legislation has already been introduced to make some kind of rear obstacle sensor mandatory, because about 100 kids are killed every year from having a car back up over them. From the rear view mirror of a Chevy Suburban, a two and half foot tall object (the height of the average two year old) isn't visible less than 45 feet behind the vehicle. Beyond cameras, other obstacle avoidance systems use ultrasonic sensors or radar.