From http://www.google.com/recaptcha/intro/index.html
Introducing the new reCAPTCHA!
The new reCAPTCHA is here. A significant number of your users can now attest they are human without having to solve a CAPTCHA. Instead with just a single click they’ll confirm they are not a robot. We’re calling it the No CAPTCHA reCAPTCHA experience. See blog for more details.