I think you may be misunderstanding my solution. I don’t want to keep weblog comments and comment forms from being indexed in Google, that can be achieved now with a robots.txt. What I want to do is allow bloggers the ability to turn off PageRank for links in comments.
Comment spammers are not the same as email spammers. Email spammers are trying to get people to read their message. Comment spammers don’t care if you click the link they provide, they are trying to get an increased PageRank in Google by getting a site to “vote” for their site by creating a link from a weblog to their site. Google uses PageRank to choose the order to display search results, and that can affect commerce sites by many orders of magnitude.
Comment spam is caused by Google’s ranking algorithm which rewards people for being linked to, combined with weblogs’ ability to allow people to post content on a site. If you want to stop comment spam, you have to change one of those two things. My solution changes Google so that spammers aren’t rewarded for getting links in comments. The other solutions would involve raising the barrier to entry for posting comments, which reduces their value.
I have no idea what a spamhouse would gain from writing their own spider. They’d find a bunch of comments, but they wouldn’t be able to affect PageRank with that information if posting comments didn’t affect PageRank, which is the case under my solution.
As for the solution on your site, it would be trivial for spammers to write a spider that would load the form, get the hidden value and include that with the comment spam. It works well if there’s only one site using it, but if a million Movable Type blogs start using it the spammers will incorporate it into their software fairly quickly.