There's not really a way to do that with this technology. These are just price tags on the shelf, so if they changed the price it would change it for everyone in the store.
Yeah, I'm not actually that worried. I've seen these in use at hardware stores for quite a while now. It's just useful to assume that Walmart is planning to fuck you over. That's a good point with BT though, many of the kind of microcontrollers that would be used for this sort of thing offer BT connectivity as well.
But you don't carry the sign with you. It stays at the shelf. Sure, they could build a system that tracks you everywhere in the store and marks what price they showed you and tells the register what price to display when you check out. They'll try all that, but this won't do it yet.
A supermarket trip may soon look different, thanks to electronic shelf labels ( www.npr.org )