We all know it's a problem.. however, I think there needs to be a pre packaged spam fighting tool... so here's my proposal to effectively eliminate all automated comment spam.
1. When a user makes a WordPress installation, a new option is added in the database... something like "commentkey" this is a randomly generated string.
2. This string is required by wp-comments-post.php as a querystring value. The form action field would then be:
action="http://www.domain.com/wp-comments-post.php?key=44JKsl30Jsl" This could easily be done with a simple edit of the wp-comments.php file.
3. This string is checked against in wp-comments-post.php if it does not exist, comment gets disqualified.
4. There would be an option in the interface to generate a new key, should spammers custom write a script for their specific installation.
While this would not completely elimintate spam, it would be make it terribly difficult for spammers to get around and require some very creative coding.
So, there's my idea. Comments?