Trying to get CentOS 5 to display a login banner is not as easy as it would seem. It should be as easy as running gdmsetup as a privileged user. Under Local tab there is a section called "Welcome Message". You click custom and type in your message and close the window. Logout and hit ctrl-alt-backspace to restart X. I get no message. I try to run gdm-restart from a tty. It restarts gdm and comes back to the login term with the same themed CentOS gdm screen. I figure out eventually that in gdm-setup you have to turn off themes to get the welcome message to show up. Under the gdm-setup local tab you have to choose Style: Plain. Then close and restart.
The way I found this out was not easy as I just figured it was smart enough to know that if you choose a message to display it would know to turn themes off. I guess not. So I found out about turning themes off by actually changing the config file for gdm that CentOS uses. To do this you need to edit the /etc/gdm/custom.conf file.
In the [daemon] section put the following line.
In the [greeter] section put the following lines.
Welcome=Authorized users only!
Then just restart X (ctl-alt-backspace) or issue the command gdm-restart from a term. If you log back in after restarting gdm and then run gdmsetup you will see that themes where turned off. That is when it hit me that unless your theme has the message embedded it it you will never see your welcome message with themes turned on. gdm-setup should be a bit smarter than this.
Just a note that on Debian/Ubuntu systems custom file is /etc/gdm/gdm.conf-custom. Also, you don't need the daemon section above for Debian/Ubuntu.