One of the main reasons we were really looking forward for ejabberd 2.1.6 is the support of captcha for mod_register. This module handles in-band registration, which means the interface where users can register through their client. This interface is also abused by spammers. We hope that the use of captchas will prevent most spam registrations.
If you client does not support captcha you well see a url where you can see the captcha in your browser window.