An eagle-eyed CodeProject reader posted a comment to my ASP.NET CAPTCHA Server Control article pointing out one French developer's very thorough attempts to defeat many common CAPTCHA techniques. He compares lots of visual CAPTCHAs side by side and comments on their strengths and weaknesses. Some of the "best" CAPTCHAs have this comment attached to them: not always human-solvable. Heh.
There's also a W3C presentation from Matt May which attempts to discredit CAPTCHAs entirely, though I think their increasing prevalence is ample proof that they actually work. If nothing else, they raise the bar much, much higher for spambots. His main point seems to be that CAPTCHAs hurt accessibility, and that's totally valid:
What the heezy?!