I have added this as a new feature. To join this usergroup and have the banner under your name you need to follow these instructions:

1. Click User CP at the top of page.
2. Click Group Memberships on the left hand side.
3. Then click join Zend Certified usergroup.
4. PM me a link to your certification on Zend's website.
5. Once you are approved go back to the Group Memberships and Identify yourself as a Zend Certified user.

That's it. Follow those steps and you will have the banner under your username! :)