On a 3.0.4 installation of WordPress with Buddypress and other related plugins, I've noticed SI Captcha has an issue.
New users can't register unless the permissions for folder /captcha-secureimage/captcha-temp set to 0777, apparently. The audio captcha file is usually N-O-N-O when that happens. Setting captcha-temp to 0777 fixes the problem.
Setting that folder to 0777 seems unwise -- and it has to be reset every time the plugin is updated -- so I'm wondering what's causing that.