Doing comparison shopping by scanning the barcodes of items with your cell phone camera is a great idea which has seen a good implementation on the G1 and also some work on Windows Mobile. Woabi is another service, unfortunately specific to Germany, which allows comparison shopping using just this technique.
While their data base is not yet fully comprehensive, it does include most popular products, and the service is completely free. The service works either via an app on your phone or via SMS, with the 13 digit barcode numbers being entered manually.
If you register for the service your scans get saved, and one can even set up the service to inform you when a scanned item hits a target price point, allowing one to be an even smarter shopper.
Hopefully the service will expand beyond Germany soon, but in the mean time for our German readers read more about the service and download the free client here.
Thanks Muris for the tip.