In response to repeated complaints for readers of this blog, I changed the comments settings here yesterday, removing the anti-spam captcha. Immediately (within minutes), there was an upsurge in machine-generated spam comments. I'm currently figuring out the best way to deal with this and whether I can feasibly leave the captcha off. (Lack of the Akismet spam filter is one of the few ways Blogger still lags behind WordPress currently.)
I have always attributed comment spam to simple commercial motives, but interesting remarks by Clay Shirky reported by David Weinberger have made me think again about the motivations behind "spam".