My parents' local supermarket already has a version of 'scan as you place in the cart.' This works ok for small trips, but two-cart food sprees are much better handled by the folks in the green smocks behind the cash registers.
When I lived in the area I only used the system once because it just took too long. My mom stopped using it because she found on one trip she had effectively stolen two items by forgetting to scan them and was so horribly embarrassed she gave up on it.