Pingback 1.0 - "It basically boils down to telling a Web log when you've linked to it, by fetching the page, looking for a header advertising a pingback server, and then invoking an XML-RPC call on that server. The best thing about this idea is that unlike similar schemes like TrackBack, it is totally transparent to both users. It is also software-agnostic, so any Web logging system can implement pingback and interoperate with all other pingback-enabled Web logs." Maybe by changing the use of "documents" to resources we might have something even better, bi-directional links.
A list of feedback on pingback one week on. The Trackback documentation.