I installed CAPTCHA and that has really helped with regular comment spam.  That doesn't help with referral spam though.  If you use dotText check your feedback tab and see how much spam you have that you didn't even know about...it was a shocker to me.

Reading David Hayden's  blog today he was discussing this exact issue.  Except that he read about how Chris Frazier was dealing with it using ReverseDOS on his dotText blog.  They both say it is working like a champ and can be setup pretty easily.  Just drop the DLL in your bin directory and make a few (3) changes to your blog's web.config.

Looks like I'll be downloading and intalling it tonight.