Posting comments to this blog now works. It had been broken.
The problem was that the captcha protection was activated but the codes displaying them were not in the template. All fixed.
The problem had gone unnoticed because captchas weren't required for registered users (which I am one of). I've added this test-case for future server upgrades. Thanks to all that sent email about this. We wouldn't have noticed the problem without your help!
This is great. I've tried to comment a few times on a few stories, and was really confused about why it wouldn't let me.