One of our users recently wanted to use Firewall Builder to manage the firewall in dom0 to control traffic to the virtual servers and control access to dom0. After a bit of testing he wrote up a short how-to blog post.
Marco's Blog Post - Xen firewall with Firewall Builder
It is always fun to see users using Firewall Builder in innovative and creative ways. If you have an unusual Firewall Builder configuration let us know and we would be happy to add it to our documentation or post a link to it.