The API used in reCaptcha has moved from recaptcha.net to google.com. Everything is redirected, so this isn't an issue for most sites. However, if the site is served over https, an invalid certificate is issued for https://api-secure.recaptcha.net and most browsers won't load the script. The API addresses are now:
http://www.google.com/recaptcha/api
https://www.google.com/recaptcha/api

The verification address is: http://www.google.com/recaptcha/api/verify