Stefan wrote in Confucius: Microsoft to take 2D barcodes mainstream?:

The fact that once you print a QR code it can never be changed is a major turn off for me.

I have to respond to that;) Stefan this is absolutely not necessary! One can use a numbered code (Short Code) or even better a numbered URL Code (Short Code URL, readable with all QR Code Readers) and then you can always change the web address the QR code should point to.

Measurable Poster Marketing
You can also point with different codes to the same mobile site or resource (mp3, 3gp, jpg etc.). This way in a poster campaign for example you can check which poster got the most clicks (= successful arrivals on your mobile landing page).

PS: Kaywa provides this kind of service.