Ubuntu: How to decouple the VNC server's screen from the physical screen?

Here are simple instructions that should apply to any linux distribution that has the vncserver executable on it. In a nutshell:

1. Create a VNC server password with vncpassword
2. Start a VNC server by calling vncserver with the options you want to use for the desktop (colour depth, screen size, sharing, etc.)
3. Connect to that instance (from the local machine or a remote machine) using any VNC client and the password you set in step 1.

Start a session with vncserver. You'll have to make an executable shell script ~/.vnc/xstartup that starts some window manager and possibly some apps