OK, I don't know for sure that this works since I just tried it out but I thought I'd post about it here anyway since others can try it out (if they liked) too and we can get a better idea as to how effective this method might be :p
I've got a full posting about the solution here: http://farook.org/arc20040919.htm#BlogID754 but the basic idea is that I thought that the reason WP blogs get such a lot of spam might be because the spammers have scripts which look specifically for the wp-comments-post.php file and send it data with the right variable names. So, I modified the WP code on my installation so that the wp-comments-post.php file is given a different name and the comment variables are renamed as well. The changes need to be made in only two files wp-comments.php and wp-comments-post.php and it was fairly easy to do :)
Now I realize that if the spammer had a slightly more intelligent script which actually parsed the comments files or the main index file, this method wouldn't work and I have some suggestions for that on the above linked entry as well but my guess is that most of the scripts out there simply look for wp-comments-post.php since most people would not change WP core files. What do you think?