In Fedora 18, GDM queried realmd to display a domain login hint, when joined to a domain. In Fedora 19 this no longer happens. Upstream bug with some research: https://bugzilla.gnome.org/show_bug.cgi?id=698200
This has been fixed after gnome-shell-3.8.1.4 in Fedora 19: https://admin.fedoraproject.org/updates/FEDORA-2013-7635/gnome-shell-3.8.1-4.fc19