Well, it's not what I recommend for major sites or businesses at all, but I can only tell you that this has worked fabulously on forums that get 1 million+ view

Well, it's not what I recommend for major sites or businesses at all, but I can only tell you that this has worked fabulously on forums that get 1 million+ views a month and has been working for more than two years. I didn't expect it to be so effective, either. As rantfoil said, if you're Yahoo or GMail or some other major site, they'll fix their bot right away. Yet if you're at all below their radar, this is the sort of easy trick that can buy you some time for spam control replanning. This works great for web software that is constantly attacked by generic script kiddie bots. It's a good bit easier than switching entirely to a different system for blogs, forums, wiki, or CMS when your captcha is broken.