The only change I could find was they removed some workarounds for known conflicts. Specifically Mootools conflict fix was removed. So if Mootools is loaded on the site, and it will be on Joomla 3, then reCaptcha will not work. It's possible there's other conflicts, but as it stands now I cannot find any issues with reCaptcha on a clean Joomla + CB install so it's likely a conflict from elsewhere causing problems.