On April 28 2011 changes were made to the reCAPTCHA HTTPS API endpoint URL.
If you are not using the latest version your users might see SSL certificate warnings when visiting your site. (However, the CAPTCHA should still load normally, unless the user has restrictive security settings.)
I've released a new version of the reCAPTCHA eZ Publish extension that fixes this issue (and includes some minor changes).
Details of the reCAPTCHA changes can be found here:http://groups.google.com/group/recaptcha-announce/browse_thread/thread/c1ff6844420c9e63?pli=1