Comparing prices and finding the cheapest item has been a favorite application example over the last 10 years. I have seen the idea of scanning product codes and compare them to prices in other shops (online or in the neighborhood) first demonstrated in 1999 at the HUC conference. The Pocket BargainFinder  was a mobile device with a barcode reader attached that you could scan books and get a online price comparison. Since then I have seen a number of examples that take this idea forward, e.g. a paper here at HotMobile  or the Amazon Mobile App.
The idea of making a bargain is certainly very attractive; however I think many of these applications do not take enough into account how price building works in the real world. If the consumer gets more power in comparison it can go two was: (1) shops will get more uniform in pricing or (2) shows will make it again harder to compare. The version (2) is more interesting ;-) and this can range from not allowing the use of mobile devices in the shop (what we see in some areas at the moment) to more sophisticated pricing options (e.g. prices get lowered when you buy combinations of products or when you are repeatedly in the same shop). I am really curious how this develops - would guess the system will penetrate the market over the next 3 years...
 Adam B. Brody and Edward J. Gottsman. Pocket BargainFinder: A Handheld Device for Augmented Commerce. First International Symposium on Handheld and Ubiquitous Computing (HUC '99), 27-29 September 1999, Karlsruhe, Germany
 Linda Deng, Landon Cox. LiveCompare: Grocery Bargain Hunting Through Participatory Sensing. HotMobile 2009.