File this under community, distributed discussions, and building trails though weblog-space: Thread the needle [via JOHO], a project to provide an easy way of linking together posts across many weblogs in the same discussion, and to start and respond to threads. Well thought-out for maximum inclusion. I hope it gets built.
See also. Movable Type's Trackback implements smart weblog post permalinks that can record and process other weblog posts that point to them (MT call it "pinging". I'm not sure about the terminology). This sounds promising: "We've also developed a tool that will display a threaded view of the entries involved in a discussion across multiple blogs whose participants are using TrackBack. This will be a stand-alone tool, not contained in the Movable Type distribution. We should be releasing it soon". And the other applications they mention are also cool. The implementation method makes me a little uneasy though, from an architecture point of view, but I can't quite place my finger on why.