if you use 'kdm', then edit /etc/kde3/kdm/kdmrc
open kdmrc with ur favourite editor and change the line 'AllowrootLogin true'. its not true on default installation.
and if you use 'gdm', then edit /etc/gdm/gdm.conf
theres a similar line like above. just remove false, write true instead.
and from now on you can sign in as root ;)
cheers,
bye.