Don Park writes about an idea he calls "Conversation Categories". The idea is having a discussion on a particular topic with each participant writing in their own blog but categorising their entry as belonging to the particular conversation. An aggregator could then pick up all the pieces of the conversation.
It's discussion datalibre-style and something I'd love to implement in Leonardo.
It actually fits nicely with some of my previous ideas around trackbacks and categories, maybe even using wikipedia for URIs.
del.icio.us has got to fit in somewhere there too!