Some stats for the curious:
The guestbook at http://thecruxshadows.com/guestbook
receives a little over 300 spams an hour, and a handful of legitimate posts a month.
The ratio of spam to legitimate posts is about 55,000:1 (yes, that's fifty-five thousand to one)
The captcha previously employed was over 99% effective, which is nowhere near good enough to leave unattended.
I'm averaging 99.6% with classifyr.com
and it's still improving. Zero false positives. This is not good enough yet; 99.99% is probably good enough for a classification-only approach with this volume of spam. A lower number would work if moderately-spammy messages were presented with a captcha.