I had taken a step between upgrades to increase security. A WordPress upgrade sometime after October 24, 2008, however, took out the ability of visitors to leave comments. The WordPress upgrade wasn't compatible with the comment-security upgrade. I thought it was just an extremely long dry spell for commentators. I also thought the anti-spam upgrades were somehow working perfectly. They had been working extremely well before but not perfectly.
Well, I finally nosed around, discovered the problem, and fixed it.
I apologize to those who attempted to leave comments during that period (with the exception of spammers to whom I don't apologize of course).
Thank you for understanding.