MacBook unplugged from external monitor thinks it’s still the Secondary Desktop

I've got a MacBook Pro 13" with Lion plugged into an external VGA monitor. Often, when I unplug its video dongle deal, the MacBook doesn't realize the other monitor is gone. It continues to believe it's the Secondary Monitor (which I can confirm by right-clicking the desktop and selecting "Change Desktop Background…").

There's no menu bar, so I can't get to my Display icon to Detect Displays. No, I don't want the MacBook screen to be the primary when I'm on the external.

Plugging the DisplayPort dongle back in does nothing. It won't even re-display on the external monitor it was hooked up to before, so no mirroring kludge if I forget. To this point, I hard power down and restart.

Any idea how to tell the MacBook there's no external monitor and that it's the primary now?