Akismet has caught 52.000+ spam comments since I installed wordpress. That was a year and a half ago, which is pretty impressive for a non popular blog.
Most of the comments come from pingbacks, so it's a good idea to install some kind of plug-in which disables pingbacks on posts after a certain time (~3 months), or just run a query to disable all comments from time to time, etc.
But I just got an e-mail with the definitive solution, and you don't need to install anything. See by yourself how easy is it:
hello , my name is Richard and I know you get a lot of spammy comments , I can help you with this problem . I know a lot of spammers and I will ask them not to post on your site. It will reduce the volume of spam by 30-50% .In return Id like to ask you to put a link to my site on the index page of your site. The link will be small and your visitors will hardly notice it , its just done for higher rankings in search engines. Contact me icq 45452xxxx or write me texxxxxxxx(at)yahoo.com , i will give you my site url and you will give me yours if you are interested. thank you
I feel very relieved now that I found the solution... I think I'm going to answer him immediately, so he can speak to the spammers and tell them not to spam my site. He's such a nice guy!