A really interesting article on QR codes.
I am not sure how many people ever actually use them. Certainly my HTC did not come with a bar code reader on it. One of my customers ran an ad with 20,000 copies and got 6 scans.
But if you are willing to assume that your user has a bar code reader and is willing to use it this is a really good article about the possibilities.
Getting the Most Out of QR Codes Using URI Schemes