In the UK one of the supermarkets did something a lot like this. (Asda? Sainsburies?) I'm not sure if they are still running it or not. Anyway, trusted customers could pick up a scanner as they went through the door. As they put stuff into their trolley (aka cart, if you are in the US), they scanned it. They went through a special checkout, where the scanner was read. There was a random check to match the scanner to the contents.

Also, in a similar vein: http://news.bbc.co.uk/2/hi/business/3712261.stm