When a technology is mature, somebody finds out how to exploit it to increase the biggest business in the Web, which is advertising. Since the first times of CAPTCHAs (Completely Automated Public Turing test to tell Computers and Humans Apart), first used by Altavista for protecting the "Add URL" service and avoiding Search Engine Spam, the techonology has evolved to commercial services that promise to convert your Web form protection into revenues.
All these services are based on a simple idea: substitute the text of the CAPTCHA by a commercial message (your slogan, etc.) in a richer layout (more "advertising"). The attention paid to solve the challenge is re-directed to the message, the advertiser gets a click (and possibly more sales), and the form publisher a part in the business.
Some of the available services include:
Still not widespread, we do not know how long will keep these services unbroken -- it is amatter of time they get the attention of hackers and spammers; or researchers :-)