When choosing to allow SSH connections in the installer, Network-daemon is supposed to setup network in dom0. Every now and then this process silently fails.
UIVM will report network connectivity (NDVM network is setup successfully)
Guests will have connectivity as well (brbridged, brshared or brwireless).
Starting udhcpc -i eth0 will solve the problem.
Rebooting the platform might not reproduce the symptom.
This can happen after any reboot.
Install OpenXT 10 or later and enable SSH
Check network connectivity in dom0 terminal