The comment spammers are going manual, it seems
Some time ago I modified this blog softare (Drupal) to ask a very simple question of people without accounts posting comments. It generally works very well at stopping robot posting, however the volume of spam has been increasing, so I changed the question. Volume may have dropped a touch but I still got a bunch, which means the spammers are actually live humans, not robots.
It's also possible that asking natural language questions (rather than captcha style entry of text from a graphic) has gotten common enough that spammers have modified their software so they can figure out the answer once and easily code it, but I don't think this is the case.
What's curious is that my comment form also clearly explains that any links in comments will be done with the rel=nofollow tag, which tells Google and other search engines not to treat the link as a valid one when ranking pages. This means that, other than readers of the blog clicking on the links, which should be very rare, these spams should be unproductive for the spammer. But they're still doing them.
The change however was prompted by a new breed of comment spam, where the spammers were copying other comments from inside large threads, but inserting their link on the author's name. (This also uses rel=nofollow.) Indeed, such a technique does not automatically trigger my instincts to delete the spam, but they chose one of my own comments, so I recognized it. Right now my methods cut the spam enough that it is productive to manually delete what gets posted, though if the volume got high enough I would have to find other automated techniques.
(Drupal could of course help by having a much easier to use delete, including a 'delete all from this IP address' option.)