The latest version of the Amazon Mobile application for iPhone adds the ability to search by reading bar codes, a feature that should really help speed up the process of ordering things online while shopping for ideas in a brick and mortar store - just in time for the holiday shopping season.
The only new feature added to version 1.2.8, which only works on iPhone 4 and iOS4-enabled iPhone 3GS, can be found under the Search tab at the bottom. I simply needed to click "Scan a barcode" to pop up the camera interface with on-screen guidance to help correctly position the lens for best image capture. It took a couple tries to figure out the proper distance to hold the phone from the product being scanned, how to avoid glare but keep the image in focus, and how to properly center the view on the bar code itself.
Once the device can correctly read and interpret the bar code image, the app automatically captures and sends the bar code to Amazon search. Connected to a fast WiFi network, the app returned the correct results in about a second. I expect the bar code search should also work over AT&T's network, although since I quit the service earlier this year, I couldn't verify that.