From the site:
reCAPTCHA improves the process of digitizing books by sending words that cannot be read by computers to the Web in the form of CAPTCHAs for humans to decipher. More specifically, each word that cannot be read correctly by OCR is placed on an image and used as a CAPTCHA. This is possible because most OCR programs alert you when a word cannot be read correctly.
Digitizing works from Internet Archive right now. It will be interesting to see how this compares to similar work with Mechanical Turk. There’s an obvious question which they also answer:
But if a computer can't read such a CAPTCHA, how does the system know the correct answer to the puzzle? Here's how: Each new word that cannot be read correctly by OCR is given to a user in conjunction with another word for which the answer is already known. The user is then asked to read both words. If they solve the one for which the answer is known, the system assumes their answer is correct for the new one. The system then gives the new image to a number of other people to determine, with higher confidence, whether the original answer was correct.
There’s been various discussion on captcha’s and their accessibility. There’s also a nice discussion about reCAPTCHA over at Ben Maurer’s blog
Props to edsu for bringing up the link again.