Installing Fedora fc18 brought up once more the new features inherit in Gnome 3. One of which being that the dconf-editor is no longer used, because Gnome 3 has it’s own version. Here is how you can disable the user list in the login screen.
Then add the following to that file:
You must then update dconf and restart gnome:
dconf update systemctl restart gdm
You should now be required to enter a valid username when logging in to your system.