Note (By PatCons)
In F9 you were able to authenticate like root, the system used to just aware you about do that and let you go on; in F10 root is not allow to authenticate, the file where this is set is /etc/pam.d/gdm, the specific line that prevent it, is this:
auth required pam_succeed_if.so user != root quiet
Of course this is for security reasons and still you can login in text mode but in there... you should know what you are doing. =D