In Snow Leopoard and earlier, you could open the lid of your laptop after you had activated clam shell mode and the display would remain inactive until you disconnected the external monitors.
In Mountain Lion (and Lion?), the behavior is to immediately turn the display on.
My question is, is it possible to restore the earlier behavior?
Note that none of the solutions listed here actually restore this behavior. The
boot-args option is especially egregious as it disables the internal display completely until you reboot. This is far less than desirable.
If you need more detail about the specifics of the Pre-Lion behavior, leave a comment.