I figured out why the authentication plugin was not working. In Red Hat Enterprise Linux 4 there is a package called php-gd which needs to be installed to provide the GD library (Graphics Drawing) functionality for PHP. In Fedora Core 2 this was in the main php package. Now it’s been separated out into a separate package. That’s why my little captcha (that’s the thing where you have to punch in the code you see in the picture) was not working after the migration.

So now that the php-gd package is installed the captcha is working again, and comment spammers can’t post without going through the authentication mechanism.