CAPTCHAs, CAPTCHAs Everywhere
http://jimthatcher.com/captchas.htm

Thatcher, Jim
JimThatcher.com
2009

Abstract:

My business and passion is accessibility and there is obviously a huge problem with these visual CAPTCHAs. If you used alt-text on this image, alt="e3TJ6Jdp", that would be fine and very welcome for blind visitors. It would also be welcome for any computer system seeking to sign up for lots of emails. Using alt-text on the image does not solve the problem! The visual image CAPTCHA is fundamentally inaccessible. For the example above, this means very simply that Yahoo excludes people who are blind (or vision impaired) from signing up for Yahoo email accounts.