I had to read this article three times before it started making sense to me. I don't know that I fully understand it yet, but I think I am getting there. It appears as though you are creating a bridge to each physical NIC in the machine, then forcing each VM to use a specific physical NIC.
However, one thing that is somewhat confusing to me is that many articles dealing with Xen reference xenbr0. I never see that on my machine. Here is the results of my brctl show command:
bridge name bridge id STP enabled interfaces
eth0 8000.00123f3b119c no peth0
It appears to me that my bridge name is eth0. Is that correct? I think that has been confusing me.
Lastly, it appears that I could build my machines and "move" them to physical NIC's when I have them. Right now, I only have two NIC's but would like a third. It would appear that the same steps apply. I was under the assumption the NIC's would need to be present before creating the VM.