Taken from the Fedora 11 release notes, which basically applies to Fedora 10 on,
Root User disabled for GNOME Display Manager
Root user is disabled by default for GNOME Display Manager (GDM) from Fedora 10 onwards. We strongly recommend that you avoid logging in as root user and instead use su -c or sudo for running commands requiring root access. If you wish to revert this setting however, refer to http://fedoraproject.org/wiki/Enabli...isplay_Manager .