I've had some fairly serious problems with comment spam on this blog - literally hundreds of bogus comments on some days. I've been trying to keep on top of it, but it's a pain. Now, thanks to the efforts of Keith Brown, a CAPTCHA feature has been implemented. Hopefully this doesn't discourage anyone from commenting. Given that it only requires you to type in four numbers, I suspect it won't.
Please contact me if you have any problems with this feature.