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.

As root:

touch /etc/dconf/db/gdm.d/01-custom-gdm-settings

Then add the following to that file:

[org/gnome/login-screen]
disable-user-list=true

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.