The MovableType folks have put together a tutorial on TrackBack, a potentially emerging new technology for weblog based notification. I say potentially because it's unclear how widespread and popular TrackBack will become. There's currently a lot of activity among ultra-geeks (even farther ahead than early adopters), but even I'm having a real hard time getting hopped up about it, and I really like this stuff!!
In general, I find attempts to support "cross-site discussion" oxymoronic. It's akin to taking a USENET newsgroup and spreading it across n different sites, where n is large enough to make following the discussion chaotic. Contrary to popular belief, centralization is sometimes a good thing.
Memo to self: investigate CSCW literature regarding asynchronous conversation