Dave Winer has proposed You Know Me to enable threaded cross-weblog conversation by means of a central server that identifies users.
The need is real -- it would be very handy to be able to aggregate conversations across weblogs. But that approach seems like centralization and ID overkill.
* Usenet doesn't require unique IDs to thread discussions -- just a unique ID for the discussion thread and a time stamp on the entry
* Threaded cross-blog conversation is valuable enough that it shouldn't depend on a single server or service.