Take a look at this URL, and the page behind it. This is a list of all the Flickr photos with the tag “xmlns:dc=http://purl.org/dc/elements/1.1/“. Although these have been around for a while, I hadn’t been aware of this kind of tagging until recently.
Why “xml” in the namespace declaration? This doesn’t have much to do with XML. How many tags are there in the world that start with “dc:” and are not referring to Dublin Core? At least the tag declaring the namespace provides a good hook for finding things with machine tags. It’s only a small step up to RDFa from here, which is good! -m