If you are using a later version of Fedora Linux (such as in Fedora 14), you will find that GUI login to 'root' is no longer allowed from the login screen. It however is possible to re-enable this.
You will either need to do this from a user that is allowed to run 'sudo' commands or as a user of the 'wheel' group.
(I usually just login as a normal user, open a terminal screen, run command "su -" to login as root and perform the commands below without the 'sudo' part. But if you already have a user which is allowed to do sudo commands, you can simply use commands below via that that user account's terminal.)
Once you have done that, do the following in the terminal:
sudo vi /etc/pam.d/gdm
Then comment out this line:
#auth required pam_succeed_if.so user != root quiet
Then open this file:
sudo vi /etc/pam.d/gdm-password
Then comment the following line (same as the above):
#auth required pam_succeed_if.so user != root quiet
Log out of that account and get backl to the GUI login screen.
Login in as root using the "Other" option. You will need to enter the username as 'root' & its associated password. If accepted, you will receive a alert prompt stating this is a not a secure way to use the root account, but after you agree to that, you are good to go.