If you are in this, maybe you could find interesting this review of a paper from googlers to approach a CAPTCHA design, in which humans are asked to select the

If you are in this, maybe you could find interesting this review of a paper from googlers to approach a CAPTCHA design, in which humans are asked to select the right image rotation: http://glinden.blogspot.com/2009/05/exploiting-spammers-to-make-computers.html http://glinden.blogspot.com/2009/05/exploiting-spammers-to-m... As always, one of the most interesting part of truly great CAPTCHA systems is that they are advancing the state of the art in image recognition. But on the other hand we still have scams like this, and no real solutions.