cp /usr/share/doc/gdm/examples/custom.conf /etc/gdm/
# GDM configuration storage [xdmcp] Enable=true DisplaysPerHost=2 [chooser] [security] [debug]
/etc/init.d/gdm restart
apt-get install remmina remmina-plugin-xdmcp
http://www.peppertop.com/blog/?p=690
http://www.informatica.escuelaedib.com/wp/2010/10/servidor-x11vnc-de-escritorios-remotos-disponible-en-el-arranque-de-ubuntu-maverick/