For any bloggers having comment spam problems, there are links at this post addressing some of the issues, particularly with blogs running Movable Type. In a nutshell, comment spam is an unscrupulous use of the blogger's comment feature, where a robot leaves URLs to drive up traffic for certain crapola sites, or create artificial traffic, or whatever. A big source of the problem is compromised Windows machines that act as slavebots to send out the spam. Meaning some popup trojan could have gotten into your PC, causing your computer to send out spam without your knowledge while you're websurfing. The real villain here is Bill Gates--it all goes back to Microsoft's slimy tactics during their antitrust suit where they argued that Internet Explorer was an integral part of their operating system and not some detachable, independently competitive feature. By actually integrating the two things, they made everyone's PCs much more vulnerable to hackers. Way to go, Microsoft!
Six Apart, the Movable Type people, have published a rather large guide to comment spam.