I just stumbled across this RFC draft by Mark Nottingham:
This document specifies relation types for Web links, and defines a registry for them. It also defines how to send such links in HTTP headers with the Link header-field.
Interestingly, it suggests a nice balance between the need for registration of well-known types and the decentralization needs: In a rel=’xyz’ attribute, ‘xyz’ is interpreted as a URI that’s relative to http://www.iana.org/assignments/relation/; alternatively it can be an absolute URI, such as http://innoq.com/st/loves-it. I love it! Now can I please get the same thing for media types?