Server 2003: Prevent external DNS lookups (as public nameserver) from returning host records for internal IP?

Your question does not make sense. A query for an external name can NOT return an internal IP, as no legal internal IP address can be used on the Internet. If you need an internal server to handle internal machine addressing, you must NOT let any machines use external DNS, so you need to set up dhcp to set all primary DNS servers as the internal server address.