mk1004 writes: Yahoo news has an article explaining how the text-based CAPTCHA is giving way to ad-based challenge/response. It's claimed that users are faster at responding to familiar logos, shortening the amount of time they spend proving that they are human.
"The eleventh commandment was `Thou Shalt Compute' or `Thou Shalt Not
Compute' -- I forget which."
-- Epigrams in Programming, ACM SIGPLAN Sept. 1982