No more clicking "I'm not a robot" checkbox, google step ahead and officially released it's most secure and innovative Invisible reCAPTCHA servic...
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So other than saying "it's magic!", do we know how it actually works on Google's side? Is it relying on things like timing, user agent strings, lack of certain HTTP headers, etc.?
Joel if you use proxy servers to access the forms protected by reCAPTCHA you will be asked for image verifications. I think google uses the browser and trusted user IP's for this. Also, you will be asked for verification if you increase sudden traffic. I check myself both situation and it works for me.