Self-scanners have been around for several years. Market District in Pittsburgh had them in 2005 when I lived there (http://gadgets.boingboing.net/2008/09/01/review-grocery-stor...). They never caught on, and ultimately the company stopped using them in 09 (http://www.post-gazette.com/pg/09226/990741-28.stm note: this article doesn't provide any details).
The problem is theft prevention and time. Every so often (appx every 10 customers), they would randomly check every item on your receipt. You think you're done, but you aren't, they need to check every item on your receipt.
The thing that IS potentially a game changer in this regard (which the article doesn't mention) is RFID. With RFID combined with Google Wallet, the theft prevention problem can be solved much easier (ie alarm goes off if you are stealing)