Didn't work properly on my 3G. On the other hand pic2shop (which was free wen I got it, don't know about now) works like a charm.it is the best.
they kinda did this a long time ago... anyone remember the "CueCat?"
I got one free at Radio Shack a few years back...
http://cuecat.com/
It's pretty much impossible to read a 2d pdf417 barcode with the camera. Plus the speed of capturing and then decoding which usually means uploading to a server having it try to read and then downloading it is way too slow.
I'm looking at developing a scanner for the device but current apple os restrictions are preventing something like that for being developed.