Google acquired reCAPTCHA, a company that has an open based CAPTCHA technology for validating humans as being real humans. Google basically wants them to help improve their book and newspaper scanning OCR technology. reCAPTCHA uses newspaper words as the identifier for human detection. reCAPTCHA collects all of the responses from humans when they are asked to enter in what they see as the words. This data is incredibly valuable to companies that want to better learn how to read words scanned from a newspaper, which is why Google acquired them.
If you scan the WebmasterWorld thread, you will see one major theme. SEOs and Webmasters are eager to drop reCAPTCHA from their web sites now that Google owns them. One senior member said, "Now busily removing reCAPTCHA from all our sites. Google has enough beacons already. No need for more." A moderator added, "next month you're all going to get ads showing in your captcha box."
In any event, I am not sure if it is worth dropping using reCAPTCHA from most sites. Heck, they already have AdSense and Google Analytics running on them.
Forum discussion at WebmasterWorld.