I've installed Mollom. This does textual analysis of comments, hits you with a captcha if it thinks it's spam.
I'm hoping it'll be more effective/less annoying than the current captcha, we'll see.
More importantly, once you've posted more than 5 comments, it's automatically disabled - so regular users won't get captchas any more. We might need to up that threshold if we still get a lot of spam come through.