Age of Discovery: "R.V. Guha, who has been thinking about these problems for longer than XML has been around, has a suggestion that I think will be helpful, a simple usable convention for identifying real objects and referencing them in any XML document. He proposes two magic attributes with a special namespace. The first would suggest that an element was a real object and would provide a way to find it—a URL or URI whose value would be in the attribute."
The paper by Guha has already changed the names from about/resource to key/ref. Already people are changing these names. I don't like the fact that all the examples include numeric keys but as it says any URI is fine.