Well after playing with it restarting X seems to make it work fine but thats a bit annoying when you just wanna yank the external and move around. Without restarting X it just seems to think the monitor is still attached and things are not visible.
I also had the same panel problem where they will move to the single monitor but not move back when the other is reattached.
I hear the next version (intrepid ibex) will be focused on mobile computing so maybe some of these issues will be addressed.
Ill give it a try a few times tonight and let you know how things go