As reported here:
http://www.turnkeylinux.org/forum/support/20090505/network-settings-lost-reboot
Webmin's network configuration module edits /etc/resolv.conf directly on Debian/Ubuntu instead of configuring DNS via /etc/network/interfaces (resolvconf package). If the system is configured to use the resolvconf mechanism, DNS settings will be lost on reboot.
Thanks .. in the next Webmin release, any dns-nameservers entry in /etc/network/interfaces will be updated when changing nameservers.