Open /etc/gdm/custom.conf and set the Enable entry to 1 in the [xdmcp] section as following.
[xdmcp]
Enable=1
Open /etc/gdm/custom.conf and set the DisallowTCP entry to false in the [security] section as following.
[security]
DisallowTCP=false
I'm assuming you are using gnome. Restart computer and see what you get, though I think you should be good to go.
system -> Administration -> Loginscreen
But here in Fedora 11 (Leonidas)
How to install the XDMCP service in this please help
http://www.yolinux.com/TUTORIALS/GDM_XDMCP.html
http://tldp.org/HOWTO/XDMCP-HOWTO/ssh.html
http://www.idevelopment.info/data/Unix/Linux/LINUX_ConfiguringXDMCPRedHatLinux.shtml