Thanks for bringing that up, I'd almost forgotten about that.

The way I understand it, reCAPTCHA gives you 2 words to analyze, one which reCAPTCHA "knows" and one it is trying to learn about. As long as you get the word it knows about correct, it'll say you're a human. You're answer to it's unknown word is simply stored and (I'd assume) analyzed until it has enough responses to consider it a known word. Knowing this allowed 4chan users to nearly cut their response time to the captchas in half.

I'd be fascinated to hear more details (or be corrected) on how reCAPTCHA works if anyone has them.