Backend device not found?

[Rob Gardner <rob.gardner@hp.com>]

Can somebody help with what must be a really simple problem? I've got a 
stock xen 3.0 installation on a brand new Redhat ES4 installation. I've 
got all the usual stuff installed - brutils, iproute2, hotplug, etc. 
Dom0 boots fine, and xend starts up ok. But when I try to start domU 
using the command:
    xm create -f /etc/xen/xmexample2 vmid=4 
kernel="/boot/vmlinuz-2.6.12-xenU" -c
I get the message:
  Using config file "/etc/xen/xmexample2". 
  Error: Device 0 (vif) could not be connected. Backend device not found.
Appending "nics=0" to the command line makes the problem error go away, 
but of course networking doesn't work.

Two other data points: First, the exact same xen and dom0/domU binaries 
work fine on another installation. Second, I saw this exact problem two 
months ago. So I have to assume it's some kind of configuration error, 
since I don't recall seeing other people reporting this behavior.

Thanks for any suggestions.

Rob