I've just achieved this.

At the top of /etc/init/gdm.conf, replace the lines:

Code:
start on (filesystem
          and started dbus
          and (graphics-device-added fb0 PRIMARY_DEVICE_FOR_DISPLAY=1
               or drm-device-added card0 PRIMARY_DEVICE_FOR_DISPLAY=1
               or stopped udevtrigger))
with the following:

Code:
start on runlevel []
Steve.