Jeremy just released the new version of his Drupal spam module. Yes, it's true that Drupal sites suffer from spam as well -- it's just always had a comment moderation system, so you can easily place comments from anonymous users into an approval queue.
This works great at low volumes. For high traffic sites that get lots of high traffic spam...it doesn't work at all. Legitimate comments get drowned out by a flood of spam -- e.g. I have 36,000+ comments sitting in my approval queue. But, since I've been an advocate of open comments, I didn't want to close things down and force people to register.
So, tougher measures are called for, and at 10:30PM PST tonight, the first piece of p*ker comment spam bit the dust. And I didn't even need captchas. Thanks, Jeremy!