Hi all, not quite a solution but a work-around here.

If you install the simple-brightness-applet (simple brightness statusarea applet), when you view the status area applet, to the left of the 5 brightness levels is another icon which gives you the option to leave the screen on.

Effective if you have the N900 plugged in to a charger but not helpful for when you're starting the x11vnc program remotely e.g. when you leave your phone at home and can VPN into your home network!