We saw this mentioned over the weekend in the NY Times Magazine and did a little more searching. Designers (both professional and DIY) are embedding QR codes in product patterns. The two-dimensional product bar codes originated in Japan, and you can use a camera phone with decoder software to scan the design and translate a message. For example, the pattern on the cake above (from Craftzine) contains a secret note. More below.
The pattern above is by John Young (aka Tikaro), whose needlepoint design translates into the url for Wikipedia's "pillow" entry. You can generate your own QR Codes at several websites, including qrcode.kaywa.com, and use them for your own craft projects.
To read the NY Times Magazine article, click here.