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A German startup is doing this, too: http://www.captchaad.com/

IIRC Microsoft even tried to patent this. The article's claim that only one company is working "as the middleman between advertisers and webmasters in order to display CAPTCHA ads" seems slightly exaggerated ;)

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There's also a Romanian startup doing this http://www.adcaptcher.com/ and I'm sure there are a couple more.

The referenced article claims the startup in question is singular.

An interesting point is the counter on CaptchaAd's homepage saying they have served over 23 million ads. They must have some data on how effective this is.

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