The default seems to be to display a “face” file for each user on the gdm login screen. We can't do this because not only we have too many users, but this will automount every single user home directory on every machine! This supposedly disables it:

dbus-launch gconftool –set /apps/gdm/simple-greeter/disable_user_list –type boolean false

I still don't get a login prompt, and I still get all the user home directories mounted. argh.