I've been needing to be able to remote desktop my new Ubuntu machine from the login screen and information on the net is always confusion. So here I will share my solution I ended up with. This is how you access your machine over VNC from the login screen with Ubuntu 13.10 which uses LightDM for the display manager.
apt-get install x11vnc
/etc/init/x11vnc.conf file with the contents as below (ending at end script):
start on login-session-start script x11vnc -xkb -noxrecord -noxfixes -noxdamage -display :0 -auth /var/run/lightdm/root/:0 -rfbauth /etc/x11vnc.pass -forever -bg -o /var/log/x11vnc.log end script
Run this in terminal replacing PASSWORD with the password you want. Use a space at the beginning to not record in bash history.
sudo x11vnc -storepasswd PASSWORD /etc/x11vnc.pass