This probably makes for a poor quality (read: weak) captcha, but it's a neat idea.

reCaptcha seeks to block spam while handling the challenge of digitally scanning old books and making them available in Web search engines.

When character recognition software fails to decipher a word scanned in a book — when the page is yellowed or the letters are smudged, for example — reCaptcha makes it part of a captcha. After the mystery word has been verified by several people, it is fed back into the digital copy of the book.

(Description from NYT article)