Decided to restart from scratch, re-installed Centos 5.6, verified network works fine, confirms no hardware or network problem causing this. Installed Xen with yum as per the howto, rebooted and again no network access.
modprobe/lsmod show the NIC hardware
ip link shows NIC up
ip route shows correctly "default via 192.168.1.1 dev eth0"
What am I missing here?