ShopSavvy surely must keep brick-and-mortar retailers up at night. If you find an item in a store, you can scan its bar code using ShopSavvy and your phone's camera. Then watch gleefully (or in horror if you're a shopkeeper) to see where the item is available online and how much cheaper it is. Buy it right from your phone, walk outside, and that's that.
See also: pic2shop, Barcode Scanner, QR Droid