DNS Servers are generally set using DHCP. There is a Group Policy to set DNS Servers which some people often suggest, but that policy is rather pointless in my opinion; if DNS settings are incorrect, Active Directory Domain Controllers cannot be located by workstations (due to incorrect / no DNS server settings), and thus Group Policy cannot be applied to get those DNS Server settings.
Run DHCP on one of your Server 2003 servers, and turn it off on any routers / firewalls. Using Server 2003 DHCP gives best flexibility, and allows for you to set options such as DNS, DNS Domain Name etc.
-tigermatt