A little background. A datamatrix is a 2 dimensional barcode, that can hold up to 2kB of data. I created one at semacode.org, containing the URL to my blog. 
You can make a photograph of this image using your mobile phone. If you have a datamatrix reader installed (install one over the air using your Java enabled mobile phone), it will translate the datamatrix into the URL. Obviously, this process only makes sense if you print the image first 
At Semapedia.org you can create a datamatrix (‘tag’) containing links to Wikipedia articles. Print the tag and attach it to e.g. the Guggenheim Museum, New York. Anybody that photographs the tag will see the wikipedia entry.
The community around Semapedia also photographs tags and its surroundings. There is a geotagged Flickr stream, a Google Map and an easy way to contribute.