It appears that the site's anti-spam software may have deleted a legitimate comment today. Out of three spam comments it has caught to date, one was legitimate (which I spotted in time), one was spam, and one I have no idea about because it was deleted before I saw it, which should never happen. I've now removed the anti-spam software, but if you've posted a comment which didn't appear on the site then please post it again! Sorry for the hassle.