The NHTSA has decided to delay the regulation requiring backup cameras until 2015, announced in a latter to Senator Jay Rockefeller from Ray Lahood. One reason stated was that NHTSA needed more time to study the cost/benefit equation for the backup cameras.
My issue with the proposal is that it will force carmakers to add expensive LCD displays to vehicles which may not already include them, driving up the price of the lower feature vehicles.
If rear ultrasonic sensors are cheaper, I would suggest that method as an acceptable alternative. The backup warning systems have the added benefit of reducing accidents that happen when people aren't careful when reversing the vehicle. In theory, a backup warning mandate would reduce insurance costs.