Comment 8 for bug 315090

I personally would model closely after the laptop Fn-F7 method, this is what users will expect as functionality. I think the most frequent one is cycling through: Single (laptop)->Mirror->Single (extern). I would suggest just adding Extended Desktop (i.e. disper -e) to the cycle (maybe as an option).
This would be perfect to be bound to the keystrokes (for my laptop it's Fn-F4 btw), or implementation as a Gnome-panel plugin. This also could contain a simple menu for configuring the cycle options (like many laptop manufacturers have a small applet to do configuration)