I am running 10.8.2 using a current macbook pro with an acer external display. The mirroring and extended desktop work fine. However I would prefer the external display to have desktop 2 open ie. with the dock and menu bar. This would allow me to selectively open apps on the native display and graphics intensive apps on the acer monitor.
Just use the Displays preference under System Preferences and drag the menu bar from the MacBook Pro display to the external display. Note that if you use clamshell mode, the menu bar will always be on the external display.