I noticed on my father's car (factory option) that his back up camera is aimed far lower than I thought it would be, and works very well that way. I was thinking if you got a camera small enough, which is quite easy, I bet you could fit it in the lighting opening for the rear license plate. You'd probably have to widen the opening so the light and the camera could be mounted effectively. But otherwise, I think it'd be a pretty stealth install.
I was also thinking it'd be pretty slick to punch out the Saturn emblem, put a tinted window into it, and a camera behind that. But a camera mounted as such would probably be more of a rear view mirror type view then a back up camera view.