Can I really do that with the phone? Perhaps I would have to call any of the native API's, but I am not really sure whether and what is possible. I am pretty sure that the camera is accessible, so at the worst case I could send the picture to the server, decode the QR code on the server, and get the results on the phone, right ?
There's a QR code reading class here: http://www.libspark.org/wiki/QRCodeReader/en and some instructions on how to use it: http://blog.jactionscripters.com/2009/05/23/introduction-of-qr-code-reader-library/
While I've never done it, iOS does support the camera as far as I know so it should work just fine. The code is pretty similar to using a webcam.
And here's a library which claims to be improved: http://www.kasperkamperman.com/blog/flash-qrcode-reader-as3/