Fonte: http://linuxers.org/quick-tips/fedora-12-enable-root-login-gui

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Fedora 12 Enable root login in GUI
Written on December 12th, 2009 at 20:27 pm by chia

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Fedora 12 uses pam module called pam_succeed_if.so. This module is designed to
succeed or fail authentication based on characterstics of the account of the
authenticated user. Default configuration of this module blocks root login
using GUI.

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Login as Normal User.

Open the terminal and use vim or gedit to open the /etc/pam.d/gdm

    su -c "vim /etc/pam.d/gdm"

Comment the following line  'auth required pam_succeed_if.so user !=root quiet'

    # auth required pam_succeed_if.so user != root quiet

Do the same for "/etc/pam.d/gdm-password

Save the file and logout from GUI. Now you can login as root :)