Xen by default will generate a random mac address for virtual machines network devices. Thanks to udev trying to keep persistent names based on mac over a reboot eth0 can magically become eth1 which sort of defeats the purpose. :-) I'll have a patch against udev 103 coming shortly to fix this.
Created attachment 107406 [details, diff] Patch for 75-persistent-net-generator.rules Adds SUBSYSTEMS!="xen" to the rule.
yeah, the new persistent stuff is actually kind of annoying
Applied patch to udev-104-r6.