Some additional references on this subject include:
Your local xdm man page.
Your local gdm man page.
XDMCP Documentation (Compressed PostScript file download)
Why Port Forwarding?; Port Forwarding; Secure forwarding of services with SSH
10 minutes to an iptables-base Linux firewall; iptables command introduction
Securing Services on your system (Debian)
Remote X using KDM (Caldera)