Obviously there are privacy concerns. That being said, this looks like a boon for anyone interested in bot detection, as you can periodically challenge your users' humanity without getting too much in their way. Nice one, Google.
From the thread:
Implemented it successfully for a website. I have to say, it works great!
it also checks if html pages are changed at runtime and how many times you "reload" the page where the captcha is. When it thinks you are a bot a captcha popups, when entered, it got checked on googles servers if it's right and fills in a hidden input. When the user submits the form, the filled in captcha coded, again, will be verifed. [sic]
[0] http://www.google.com/recaptcha/api2/demo
[1] Edit: don't go to this url without adblock (see comment below). http://forum.ragezone com/f144/googles-captcha-recaptcha-1023607/