I've been thinking about this for a while, (probably more than most, given the time I've spent on plugins) and if you really want to venture into tin-foil hat country, the spam problem might even have deeper roots than it seems.
Observation #1: Very few blogs have visible spam, regardless of what's piled up in the mod queue, visibility counts.
Observation #2: People (site owners, not necessarily the actual spammer) promoting a business tend to look for results.
Observation #3: IP log analysis from Spaminator results shows a large variety of IP addresses used.
The average joe that wants to make money with his website, doens't have the resources/knowledge to spam a large amount of blogs. He wouldn't be able to vary his IP so drastically, either.
So, he has to hire someone, the spammer, with his spambot software and access to zombied proxies to send it from.
The spammer has a nice scam going, and he's working both ends of the table. One, the site owner sees a need to "get the word out" and the spammer is the go-to guy for that, supposedly. Also, the spammer doesn't care about his comments are moderated, because he's not looking for results, he's just looking to show that x amount of comments were posted and he can get his fee.
So the site own has been duped into thinking that he's getting pagerank, but really only paid for his door hangers to get tossed into the dumpster. If he doesn't get pagerank from the spamming, he's unlikely to continue. The spammer must find more paying "victims."
This explains the spam that comes in spurts, the site owners don't get results, and give up. But what about the consisten spam that pours in rain or shine?
On example from my site is the 'byob' spammer, (if you've been hit, you'll notice that signature in the email address used in the comment). He's been attacking my site for over 2 months, and has been directly responsible for several improvements in Spaminator.
I can only come up with two reasons. He's got his own botnet, and is trying to "make it up in volume," or he has another plan in mind.
Could spammers like this be working to simply harass blog owners? With the goal in mind of damaging part of the blogosphere to the point that comments are shut off on a majority of blogs?
I have no idea why, but it seems like a good of explanation as any. Blogging is disruptive media, and maybe we're under an orgainsed attack. It's hard to say, but it's a theory that I've not seen discussed before.