it will set you up to span if you plug in the dongle, with or without monitor. Actually, this is probably just for the 12" as the larger models have full DVI.
don't forget you can drag your menu bar over to your other monitor in the preference panel. This will allow you to use the large monitor as your primary display. OS X will keep this preference and move your dock and menus back to your monitor every time you reconnect. It really is easy and works flawlessly.