PROBLEM

In Fedora 10, the root login was disabled into the GUI, giving you a unable to authenticate user message.  You may have noticed that you got a warning about this in Fedora 9 but it wasn't yet disabled then.  Not so in Fedora 10 so this might be something you would want to do prior to rebooting after upgrading to Fedora 10.  I'm not all too surprised this eventually happened.  It's not uncommon to hear of logging in as root in the GUI being discouraged whereever you happen to ask.  So essentially the pot finally boiled over and we no longer have this option standard at least on the latest Fedora releases.  This effects Gnome, KDE and the whole lot.

Essentially the problem relies in the GDM file and here is how to bring this back:

SOLUTION

Anyway, here is how to enable or reenable it should you run into the same problem.

1. At the UI login screen hit CTRL-ALT-F2 to switch to a ready terminal login screen.
2. Login as the root user ID.
3. cd /etc/pam.d/
4. cp -aR gdm gdm.old
5. vi /etc/pam.d/gdm
6. Comment out the line like this (add item in green):
   # auth       required    pam_succeed_if.so user != root quiet
7. Type :wq to write the file out and quit.
8. Since Fedora 11, you may need to edit another file still, /etc/pam.d/gdm-password to allow for root logins.
9. Again if you need to edit gdm-password, comment out the below line like this (add item in green):
   # auth       required    pam_succeed_if.so user != root quiet
10. Type CTRL-ALT-F7 or F1 to return to the login session.
11. Try to login as root again.

Of course there could be other issues.  Possibly you may need to reset your password and if you are trying to edit the /etc/pam.d/gdm or gdm-password files from within the UI as an unprevilidged user, then you'll also need to su – or sudo as root.  If the above doesn't work, you'll need to restart the system and login using the root account in single user mode.