As a brief follow-up to my post about dealing with spam on Drupal, the problem I had with Mollom turned out to be purely in my site configuration. Apparently some of the configuration changed from the previous version I had been using, so as soon as I re-saved the settings it all started working correctly again! I'm happy to say I haven't seen any spam come in during the past ten days and Mollom is the only anti-spam engine I'm currently running.