Well, I’ve got to say, Drupal’s captcha module is a huge disappointment. The idea is good but the execution is terrible. It caused serious confusion for the session management mechanism, causing people to be randomly logged out, or even sessions confused between different people. That’s a huge security risk, and just made the site so unusable that I couldn’t keep it on.
The good news is that I upgraded to Drupal 4.6.0 in all of this, and the new spam module looks much nicer than the one in 4.5.x. So maybe this will keep me happy for awhile.