Starbucks has launched what it calls MyStarbucks, an iPhone app that allows you to purchase drinks by just having the image on your phone scanned at the store. While still in beta and just in 16 locations on the West coast, it is an intriguing idea. This supplements their "Starbucks Card Mobile" iPhone app, where you can transfer the balance on your Starbucks gift card and get a $5 bonus the first time you reload it with additional cash.
This is a very fertile field of development, both for pay-for-purchase and also for data retrieval applications. China has developed a standardized bar code called Tcodethat is used on insurance cards and printed advertisements from a company called GMedia. When users scan the code on their phones, it takes them to the appropriate Web URL. This avoids a lot of extra typing, especially given what you have to deal with the Chinese characters.
Another active development area is Japan, with similar data entry problems to China. They have standardized on their own 2D bar codes called QR, and most cell phones in Japan can parse these codes and act on them in a similar fashion. Google's zxing project has a series of open-source libraries that support this format on a variety of situations.
Finally, Jeff Sharkey wrote an Android app that scans ordinary UPC bar codes that will locate the item and find where the item can be purchased and get driving directions or call the store to see if they have it in stock.