I absolutely LOVE the "attached sidecar for the cashier" idea if one gets to pick the cashier. I rolled off my chair laughing when I read that!
As an X-Navy guy, I swore on discharge that I would forever avoid all lines from that day forward. So,... I really REALLY like this idea. I always use the do-it-yourself scanner at my local Harris Teeter grocery store but never thought about having one on a shopping cart. I think that with an inexpensive strain guage (1/100th oz. resolution) built into each shopping cart, along with the scanner (with RF/infrared link for price updates) and some clever security sensors, this might be doable.
AND... I just remembered; there is a company just now marketing a new "RF barcode sticker" that works on proximity of the product. If you remove this sticker it self destructs the bar code. The scanner would continuously scan everything in your cart looking for any changes (swaps) and remain current all the way to the register. A complete update of everything in your cart would occur about every few seconds and update your LCD screen. Store door sensors would also be able to detect unpaid items in a customers pockets. In fact, if you have an item in your pocket, the focused register sensor will automatically add it to your total. The stickers cost less than one cent. This would also eliminate the need for the strain guage and security sensors. Now, if anyone can figure out how to make one of these RF stickers 'edible' so one could be injected into the center of fruits and vegitables, this might be a viable solution.
OH,... and be sure to stick one on your kids before they leave the house!