UPDATE
Nothing lasts for ever. I just restarted gdm3 (July 29, 2012) and my list of users again popped up.
None of the tips in my original blog or the ones suggested in the comments made any difference.
With help from dkg's grepping ... we found in /etc/gdm3/greeter.gsettings:
#disable-user-list=true
If you don't have that setting, you can add it under the [org.gnome.login-screen] section. If you do have it, just un-comment it.
No need to dpkg-reconfigure, just restart gdm3.
Also, don't be fooled by IncludeAll or Include in /etc/gdm3/daemon.conf. Those settings don't seem to have any affect.
INFO BELOW IS OUTDATED
I don't want to advertise the list of available logins when gdm starts and presents a login screen on my laptop.
Seems harder to figure out how to do this than it should...
Thanks to a helpful blog I've figured out two ways to accomplish the task:
sudo gconftool-2 --direct --config-source xml:readwrite:/etc/gconf/gconf.xml.mandatory --type Boolean --set /apps/gdm/simple-greeter/disable_user_list True
or edit:
/etc/gconf/gconf.xml.mandatory/%gconf-tree.xml
And add:
<?xml version="1.0"?>
<gconf>
<dir name="apps">
<dir name="gdm">
<dir name="simple-greeter">
<entry name="disable_user_list" mtime="1338752251" type="bool" value="true"/>
</dir>
</dir>
</dir>
</gconf>
You could edit /etc/gdm3/greeter.gconf-defaults and then issue a dpkg-reconfigure of gdm3, which is my preferred method.
Just fyi in gdm3 you can achieve the same by setting this in the /etc/gdm3/greeter.gconf-defaults file:
I imagine that having the user list is useful for tablet touchpad systems. But in general I would not want to have it that way and certainly not for desktops. At least not if security is needed. And if not then an automatic login with kiosk mode is much more convenient.
Editing/etc/gdm3/greeter.gconf-defaults should be sufficient. No need to dpkg-reconfigure.