Jeff Wilcox - the man who I've described many times as the "Gandalf of Windows Phone Development" - has just done a huge favour to all developers out there and released a control library for Windows Phone which will find and read QR codes.
That's no small feat, the control offers a "camera preview surface" which will then take care of QR reading for developers out there who'd like the capability, but don't quite know how to go about image processing.
If you're not familiar with Jeff, he is a senior software development team lead for Windows Phone, who helps out the development community in his spare time, managing the Silverlight toolkit for Windows Phone which every developer should be considering in any app development they approach. Of course he is also reponsible for the wildly popular 4th and Mayor Foursquare application.
Jeff is working on a blog post to explain the control (we'll update this post when it's available) but in the meantime you can grab the library and familiarise yourself with the ReadMe file at the source link.
Update: sure enough, here's the blog post with details on how the control works, as well as how to implement it in a simple app