JavaScript QR Code Reader - can it be done? Or, Remote Service?
I bet it's possible, but it would be a challenge. Someone's written an AS3 library for reading QR codes. I'd start by reading up on image manipulation in Canvas.
If you go down the remote API route, Kaywa have an API you may be able to use.
You can use the getUserMedia API to get video from the webcam and you could put it into a canvas element and use the canvas to read the pixels and decode a QR code.
I don't know of a library to decode QR codes but here is one library that can do bar codes.
There's a javascript library already, however the comments are mostly in Japanese and there's no documentation.