I think there are a lot of good points on both sides of the issue for this mandate—it will be interesting to see the magnitude of the reduction in backover accidents. As someone pointed out, if your mind is off, a camera won't help, yet if you're overly focused on the camera, side impact accidents will rise.
I think the point that opponents need to remember is that no matter how vigilant one is, it can be
impossible to see a child behind a vehicle due to the blind spot, which tend to grow as cars become safer with higher belt-lines. This is not a mandate for a safety device in response to a problem largely stemming from driver inattentiveness, which I would be more likely to have a problem with (i.e. if something like Volvo's pedestrian system were mandated...although I'm sure that's coming)
The "I'm a good driver, this couldn't happen to me" attitude is ridiculous for a backover and the question, "What if I ran a child over?" needs to be asked. If you ran over your kid, wouldn't you have wanted some way to know he or she was there? If you were told it would cost a $100 per car to have had some way to see in your blind spot, would you argue that was too high of price to pay? Plus, we tend to focus on deaths; let's also mention the 17,000 injuries annually, many of which probably require hospitalization ($$$).
As for the economics of it, I say they're overblown. They're already standard on 45% of vehicles, after being a rare option, usually bundled with NAV, just 5 years ago—without any mandate at all they likely would have been standard on the vast majority of vehicles in five years time anyway.
There are some parallels between the evolution of safety systems in cars to the airline-manufacturing industry, where more sophisticated automation and fail-saves HAVE made air travel safer, yet when accidents do happen, we are reminded that it can still all come crashing down when pilots receive inadequate training (in high altitude stalls, for instance)or have gotten away with shutting their brains off one too many times.
So I tend to support the mandate, but would strongly support some measure of collision avoidance/spin recovery/inclement weather driving to be taught in driver's ed (check out Finnish driver's education system:
http://www.youtube.com/watch?v=wnYGhAXbDsM) . It's amazing to me that we give out licenses to people who have never had a safe opportunity to attempt even an emergency lane change.