This is a pretty long video (50 some minutes), but it's worth the time to watch. It discusses these issues, and more.
- What is a Captcha, and why is it not the ultimate protection against automated attacks?
- Why do I see a Captcha sometimes, when I'm looking at pictures?
- Why are there so many free online games?
- How does Google Images get their pictures labeled so accurately?
- How do hackers and spammers setup multiple online accounts, using scripts, even with Captchas required by the online accounts?
Captchas and Online Games: Human Computation (Luis Von Ahn: July 26, 2006)