I have to second your hunch: "Especially as advertising revenue streams in, there is every incentive to post about sexy subjects that are likely to bring in search engine traffic and only marginal incentive to write complicated, informative stories about subjects of concern to a relatively small audience."

I work in the Internet marketing industry, and can assure you that the industry is very much aware of this fact. I've seen a number of articles promoting the use of blogs as a means of attracting Google AdSense income for the blog publisher.

The strategy's core is to use existing tools to identify high-traffic search phrases that also attract big-bucks bids on Google AdWords (the advertiser side of Google's ad network). Then, build a blog around those terms and watch the bux pour in from Google.

However, it takes knowledge and commitment to prosper, which ultimately should settle things down. One guy who built a blog on "mesothelioma" admitted he'd abandoned it, as there was only so much he could write about, and he didn't want to become an expert on the subject.

Still, the recent meteoric growth in blogs is due not so much because so many people have so much to say, but because shrewd marketers have learned that blogs are good at 1) getting high search-engine rankings for their existing websites, via inbound links from keyword-saturated spam-blogs; 2) attracting lucrative clickthroughs from ads placed on blogs discussing pop topics.

The first strategy has spawned a lucrative cottage industry teaching the "blog-and-ping" method, complete with software, instruction manuals and email lists for those who can't figure it out for themselves. Google is aware of this abuse of the blog philosophy, and is taking measures to identify and penalize spam-bloggers' Google rankings.