I thought some may find the following interesting, taken from another list i'm part of.
Hello group members,We are a research group from a university, TOBB University of Economics and Technology, Turkey. We developed a new CAPTCHA system, which we believe is both accessible and usable. In order to test this argument, we prepared a website in which we kindly ask you to solve a total of 6 CAPTCHA tests and answer a mini survey about these systems. Average completion time for this study is 10 - 15 minutes. You can access the website through this link:
http://www.smartcha.net/chi_usabilityWe appreciate if you could participate in this study. All your comments are welcome (either on web page after the mini survey or as an email).
Thank you.