I don't claim to be an expert on these things, but I have stumbled on a set of techniques that have been 100% accurate for preventing comment spam on this blog. And, no, it's not security through obscurity, smart ass.
I log every comment form submission, spam or not. Since I implemented my system a month ago, I have had 189 submissions. Of those, 11 were legitimate comments. The rest were spam.
All of the legitimate comments were published immediately. None of the spam comments were published. Not bad, eh?
As promised, the following slides explain in some detail the system I've implemented. I welcome your comments on these techniques below.