Start listening on 5901
tightvncserver -nolisten tcp :1
Stop listening
tightvncserver -kill :1
Tightvnc is easily installed though the ubuntu package manager.
I choose ultra vnc for my windows client to connect.
For actually sharing the active desktop use x11vnc
x11vnc -safer -forever
The forever is required if you want to allow connection after connection. You can add this to the gnome start up programs if you like.
x11vnc did not seem snapy enough across wifi to make it worth while.