I'm seeing these around a lot more and did some quick research. All-knowing wikipedia.org says:
A QR Code is a specific matrix barcode (or two-dimensional code), readable by dedicated QR Barcode reader and camera phones. The code consists of black modules arranged in a square pattern on a white background. The information encoded can be text, URL or other data.
Common in Japan, where it was created by Toyota subsidiary Denso-Wave in 1994, the QR code is one of the most popular types of two-dimensional barcodes. QR is the abbreviation for Quick Response, as the creator intended the code to allow its contents to be decoded at high speed.
Here's a QR code for my most recently released book. If you know how to use that with your "smart" camera phone (iphone, etc.) then you can check it out (I guess?!).
Is there an application here for dental marketing? Yes. However, consider it a potential distraction for now, until this technology proves itself. It's technology only a small segment of your potential patients are going to likely be "up to par" on. I don't think it's a game-changer at this point, but time will tell.