I configured my docked Ubuntu laptop with an external screen to have an extended desktop using Xinerama. It works (almost) nicely. I have a wide desktop that I can drag screens around on now, but unfortunately the pointer isnt rendering properly on the external screen – instead of a clean pointer I have a 2cmx2cm square on the external monitor. Even with a clumsy mouse pointer, this is an improvement on my previous dual monitor setup which used aticonfig’s bigdesktop setting. I found that bigdesktop configured two separate desktops which meant that I was unable to run firefox/thunderbird on both desktops, or drag windows from one screen to another.
These two references were useful:
Half Decent 