I like reCaptcha too, especially given the initiatives of the Public Knowledge Project (reCaptcha helps digitize books). This would definitely involve some legwork, but it shouldn't be too difficult if you're familiar with PHP, as displaying a recaptcha and authorizing it is essentially the same process as we use for the regular captcha.
You could create a new class called reCaptcha, and model it off of the the Captcha class in classes/captcha/Captcha.inc.php, include the PHP library provided by reCaptcha here
, and then modify the CaptchaManager class to be able to use the reCaptcha class (ideally, it would check the config.inc.php file for a setting saying what type of Captcha to use, just as the call Config::getVar('captcha', 'captcha')
checks to see if Captcha is enabled). Then, in the rest of the code where the CaptchaManager is called (and a captcha is displayed), you shouldn't have to make any modifications. Don't worry about the CaptchaDAO (which handles database calls), as reCaptcha does most of the work on their server, you won't have to worry about any database manipulation on the OJS end.
Hope that's enough to get you started,