I think it's actually not that tough to monitor relevant conversations in the blogosphere using a combination of search feeds, relevant keyword searches and RSS feed subscriptions to relevant blogs. I find that I can easily identify significant conversations about my agency, my column or my person within minutes of their posting. But there's a big chunk that we're missing. There's a lot more conversation going on, and a good bit of it is happening on bulletin boards and online community sites (particularly the ones with Open Source content management systems). Many of these sites are not indexed by major search engines, and many of them fly under the radar.
I don't mean to imply that every marketer needs to know about every online conversation involving their product or their product category. But I'm convinced that in missing out on many of the underground bulletin boards, we're missing a sizeable chunk of the market conversations. And I'm not sure how to measure how large that piece is, or what remains to be indexed.
There are other components to this, too. Some pieces are more significant than others. Chat room conversations are valuable, but probably less valuable than their BBS counterparts since they have extremely limited shelf life. No chat room conversations are hanging out long enough to influence folks other than the original participants.
What we really need here is community search, an index of sites and communities with conversational capabilities, with the one-way stuff filtered out.