I believe what you are talking about is RFID tags. Walmart did put them in, but shortly removed them from the shelves . They want to use the in warehouses... not in the stores. Just for the testing. (The last time I read about it).

RFID Chips Are Here
By Scott Granneman
Posted: 27/06/2003 at 13:17 GMT

Bar codes are something most of us never think about. We go to the grocery store to buy dog food, the checkout person runs our selection over the scanner, there's an audible beep or boop, and then we're told how much money we owe. Bar codes in that sense are an invisible technology that we see all the time, but without thinking about what's in front of our eyes.
http://www.theregister.co.uk/content/55/31461.html

Starting this week, the nation's largest discount retailer will quietly begin selling tracking-chipped products to clueless shoppers. The first volley in their war against our privacy is set to start at their Brockton, Massachusetts store.

Wal-Mart will put Radio Frequency I.D. sensors on shelves stocked with RFID-tagged Gillette products, but they'd rather you didn't know about it, because, hey, you might not like it, and then you might make noise and then they'd have a big PR mess on their hands.

You might even stop buying Gillette products or, say, refuse to shop at Wal-Mart.
http://auctionknowhow.com/articles/wmart.shtml

I have to say... since I have read about this... I have stopped shopping at walmart...
I'm all about privacy... what little privacy we have left. Soon we will have none left...