The udev stuff I mentioned before was working correctly, turns out it's something in the xen network-bridge script that isn't. The default of network-dummy works, so I can at least get some testing done. The network-bridge script is failing to copy the settings from eth0 to xenbr0, peth0, vif0.0, and not creating a random MAC address. Turns out someone on the xen-user list is having the exact same problem with Etch and xen, so maybe with a few people working on it an answer will be found soon. Looks like I'll be debugging scripts today.

A few quick notes:
I used Steve's article on http://www.debian-administration.org/articles/396, where everything seemed to work, except uncommenting the network-bridge option. ifconfig shows all of the devices that are needed by the bridge, but no IP addresses and no MAC addresses.