http://weblogs.asp.net/nannettethacker/archive/2008/03/16/spam-catch-without-captcha.aspx

Nannette Thacker, consultant and owner of Shining Star Services LLC, specializes in development of dynamic database driven web applications utilizing ASP.net technologies. Nannette has been developing ASP sites since 1997. Nannette has written numerous articles on web development techniques and tutorials.