To reproduce just sudo /usr/bin/gconftool-2 --set --type bool --config-source xml:merged:/etc/gconf/gconf.xml.defaults /apps/gdm/simple-greeter/disable_user_list true Once. You won't be able to step back anymore. disable_user_list will be set read-only and you can't reset it nor set it false anymore.
This is not a distro specific bug as you found out already. Please see directly with upstream.
Before doing that, check that it isn't already fixed in trunk.