Reading bar codes with mobile phones is very popular in Japan. It has yet to catch on in the rest of the world. Interesting read at the Economist on the topic. I have spent a decent chunk of time researching and prototyping QR Codes for use on mobile devices. When it works it is very cool. It does not work all the time. Even open source QR Codes and EZcodes have limitations. The real problem is the devices themselves. Third party providers cannot work on any and all devices that supports a camera. There are just too many phone/carrier/operating system combinations and permutations to do so profitably, and every new phone adds to the matrix. Third party application providers, like ScanLife, claim they work on 80% of phones. Naturally, ScanLife only just recently added support for my Samsung Epix that has a 320 x 320 screen. Device issues are only going to become more complex in the future; however, the camera technology that will allow for better scans will improve.
Interestingly ScanLife used to support QR Codes, but current versions do not. They use their proprietary ScanLife format. More formats, mean more issues and yet another dimension to the phone/carrier/operating system matrix.