Some additional references on this subject include:
Your local xdm man page.
Your local gdm man page.
X Display Manager Wikipedia
GDM and XDMCP Security
Using and Managing GDM
Configuring XDM (from Linux Journal)
Configuring Chooser through X Resources
XDMCP Documentation (Compressed PostScript file download)
Should you be running XDMCP?
Accessing Xterms from Windows
How to install X-Win32
Taming the X Display Manager
Why Port Forwarding?;
Secure forwarding of services with SSH
Using Exceed X Server with SSH X11 Tunneling
X11 Forwarding over SSH using X-Deep/32 and PuTTY
GNOME Display Manager
10 minutes to an iptables-base Linux firewall;
iptables command introduction
Running X Window on MAC
Securing Services on your system (Debian)
Remote X using KDM (Caldera)
HOWTO XDMCP in Gentoo Linux wiki page