I don't have a problem with comment spam on my blog (probably because only a handful of people read it), but I have read about this problem occurring on other blogs. Well I had an idea on how that might be fixed.
The idea came while I was checking out the ASP.Net Resource Kit. The kit comes with a free component from SAX.Net that does human verification.
By including the Sax.net Human Verification Component on your site, you can protect your server from these automated programs. A web page that contains the Sax.net Human Verification Component acts as a virtual gatekeeper who ensures that only real people and no automated programs fill out forms on your site.
If you've ever bought tickets from price gougers ticket master then you've seen this type of component at work. It shows you a phrase that is obfuscated so that only a human can read it. So the idea would be to put this component to work in the comment form so that you could verify that only a human was posting the comment.
Anyhow, maybe this is a dumb idea. If so just disregard it, but you should still check out the ASP.Net Resource Kit since it has a lot of free goodies (and maybe they will give you some good ideas). :)