You can also enable gdm on multiple X-servers using a simple file editor from the console by doing the following.
Open /etc/X11/gdm/gdm.conf with your favorite text editor and find.
# These are the standard servers. You can add as many you want here
# and they will always be started. Each line must start with a unique
# number and that will be the display number of that server. Usually just
# the 0 server is used.
and insert this after
at the promt and you will now have 3 gdm logon managers running after restart, CTRL-ALT-F* *=7,8,9
You can set this up to 6 so F7 - 12 switch to different x-servers.
Other benifits of this method is you learn more about GDM by scrolling through the file and reading comments and seeing the actual configuration settings. As you become more familiar with linux, you will realize that the command prompt and a text editor is your best friend when it comes to customizing and confiuring things. GUIs are limited by what the developer intends for use, the command line and text allows for much more flexablity, this can be seen with the Unix pipe system. I hope this spurs more people into learning more about the core of linux and how to configure things by hand.