Backup cameras are awesome, for, you know, not killing small children. (I guess it's even more awesome to teach children to respect and fear 2-ton steel machines, but hey, we can't have everything.)
With that said, I'd never replace mirrors with cameras using today's tech. Look, Sony makes the best goddamn sensors in the world, and they're amazing in the day, and they can be good at night... but in low light and poor light, you either get a lot of noise or you need long exposures. My eye needs neither of those things; it just works. It's the same issue of electronic viewfinder versus optical, on a camera: Optical lets me look through it at night and see stars. Some day that will change, but today is not that day _yet_.
The best solution today, I think, are electrochromic mirrors. They're great at night, because if you shine a bright light on them (ie, headlights) they tint themselves. They're common to use inside the car for a rearview, but less so on the outside.
Backup cameras are awesome, and I wouldn't mind them being required as a safety feature, or at least a cheap option, along with proximity sensors. But to replace mirrors? Not today. Maybe in ten years.