In honor of Xen Doc Day, I've put up some basic HOWTOs for setting up bridging on CentOS 6 and 7. I'm far from an expert, so I'd appreciate any feedback.
The howtos can be found here:
https://wiki.centos.org/HowTos/Xen/Xen4QuickStart/Xen4Networking6
https://wiki.centos.org/HowTos/Xen/Xen4QuickStart/Xen4Networking7
-George