CentOS - Jan 2008 - CentOS 5.1 and Xen HVM guest networking problem

I've asked this on the Xen users list, but had no response so far:

I'm running CentOS 5.1 with all current updates:

xen-libs-3.0.3-41.el5
xen-3.0.3-41.el5
kernel-xen-devel-2.6.18-53.1.4.el5
kernel-xen-2.6.18-53.1.4.el5

and have a domU config file from another (5.0) server which works.  The
difference is that on the new machine I want to use routed rather than
bridged networking.  In bridging mode the domU works fine, but I
can't use bridging on this particular server because of the external
network configuration.

I have modified /etc/xen/config.sxp, commenting out

   (network-script network-bridge)
   (vif-script vif-bridge)

and uncommenting

   (network-script network-route)
   (vif-script     vif-route)

I've also changed the vif line in the domU config file to:

   vif = [ 'ip=aaa.bbb.ccc.ddd, vifname=veth1' ]

The domU now boots, but has no eth0 interface (the driver doesn't load
in the domU because it can't find a suitable device).

Is there anything obvious I might have done wrong or missed out?  There
are no ethernet-related or network device errors in the logs that I can
see.

James