An article on CGI Security about CAPTCHA highlights a very obvious flaw in the whole philosophy of CAPTCHA. For those who are too lazy to click the link and read the 2 paragraphs the article basically describes how some very inventive worm writers enlist the help unknowing participants who are visiting pornographic web sites to play a game that offers to reveal a pornographic image piece by piece as the user breaks a code (obviously the CAPTCHA the worm is currently observing at the site it is trying to create a bogus account for).
I have to concede this is a very clever use of the Mechanical Turk idea. CAPTCHA is designed to weed out humans who are genuinely trying to sign up to a site and create accounts from computer based worms that are trying to create bogus accounts for their writers evil ends. CAPTCHA actually stands for Completely Automated Public Turing test to tell Computers and Humans Apart, and there is the flaw... the Turing test is designed to distinguish between humans and computers, if you the computer you're testing can enlist on-mass large numbers of humans to take the test for it, then the test will be rendered redundant.
This raises some very interesting concerns for identity verification. Everyone wants their site to have the smallest possible barrier to entry, even the simplicity of CAPTCHA systems can sometimes prove to be a barrier eliminating a small number of false positives. So what's the alternative? I'm not quite sure, maybe something like Windows CardSpace, or a competing identity management protocol that allows a third party company trusted by both the provider and the client to validate credentials, but of course there would have to be overwhelming take up of the same protocol set by both providers and clients which seems to me to reduce to a chicken and egg kind of problem. Also the cost of maintaining this systems integrity by the trusted third party would need to recouped from either the client or the provider. I have to confess, I'm not really an expert on identity management, so I'd love to hear others thoughts.