To set your DD-WRT router to automatically update openDNS with your WAN IP follow this tutorial -> http://www.dd-wrt.com/wiki/index.php/OpenDNS#OpenDns_with_DNS-O-Matic_for_users_with_a_Dynamic_IP

You may also use one of openDNS's clients to automatically update your WAN IP with openDNS -> http://www.opendns.com/support/article/109