Just click on the the "Scaled" radio button.
This thing is gone. I rely on it to toggle mirroring before, but it seems cmd+F1 can do the trick too.
OMG. I didn't know about cmd-f1 until I searched for this. SwitchresX put my displays in mirror mode before it started crashing hopelessly, and since there's apparently no gui for entering or leaving the mode in ML, I was completely stuck. Thanks!