XML Linking Language (XLink) is currently in committee. One day, the www hypertext won't suck:
This specification defines the XML Linking Language (XLink), which allows elements to be inserted into XML documents in order to create and describe links between resources. It uses XML syntax to create structures that can describe the simple unidirectional hyperlinks of today's HTML, as well as more sophisticated links.
The XLink Extended Links look interesting.
More interesting still is XPointer (XML Pointer Language) which provides a way to reference fragments of xml documents. The possibilities are intriguing; I could see someone building a whole document CVS on top of Mozilla with these kind of tools one day.