Tested on: Ubuntu 9.10 (Karmic Koala)
Ubuntu has a Remote Desktop server (Vino), which can be easily configured. But personally I found it to be too slow (even on 100Mbps connection) and there is no option to get it up & running before actual GDM login. One of the faster alternatives is x11vnc. It allows one to view remotely and interact with
real X displays with any VNC viewer. Following are the steps to enable x11vnc...