GDM2Setup is a graphical tool that allows us to setup the new GDM2 included since Ubuntu Karmic.
After installation, a new entrance in the menu appears: System > Administration > Login Screen (GDM2Setup)
See also my post about other graphical setup tools for the Ubuntu bootup process.
sudo add-apt-repository ppa:gdm2setup/gdm2setup
sudo apt-get update
sudo apt-get install python-gdm2setup