When i had first installed linux, i had the same problem of not being able to login as root in my graphical environment. I have kde3 and here's what i did after searching through this same forum:
Open a command line terminal window.
You'll need to change to root to be able to modify configuration files
Type in: su root
On pressing enter you'll be asked for your root password
Open the text file /etc/kde3/kdm/kdmrc
I prefer pico so i used the foll command:
You'll have to find where it says AllowRootLogin
This will be set to false...change it to true and save the file
Next time you reboot you should be able to login with KDE using your root account.
Hope this helps