One of the problems with some previous bar code scanners for the iPhone has been a lack of good close focus (helped somewhat for owners of the iClarifi case), but the new and better camera in the iPhone has greatly improved the situation. It is, therefore, no surprise that new software is rolling out to aid in your shopping by scanning bar codes and connecting you to Amazon to allow you to comparison shop.
Cyclops [App Store link] requires the new 3GS camera, and in my tests it worked pretty well. It correctly identified 9 of 10 products on the first try, although I was doing the test at home under pretty good lighting. I tried some poorer lighting and mixed tungsten and sunlight, and it did 6 of 10 scans correct. The challenge is still to hold the camera steady and try to make sure you get a sharp image, but the software did pretty well even with imperfect focus.
The software is free, so it is not a very risky proposition. I can't guarantee you'll be popular at a retailer while you price check, but I think it most cases the software will work for you. Be careful to get the distance right so the bar code pretty much fills your screen. It gives the software the best chance of interpreting the scan.
One weakness is you can't get the info out of the program easily. It would be good to be able to email the Amazon data to yourself or others.
If you need this sort of information, and don't mind nasty looks from store clerks, I'd suggest you give it a try.
Here are some screen shots as I put the program to work today: