I was also frustrated. Using 10.8.2 on my MBP 7,1, the sudo command to change the nvram boot-args worked for me for a while (from 10.8.0 through 10.8.2s), but for the past few days I could not get my built in display to be dark with the external display connected. I LOVE my trackpad and want to be able to use it when an external display is connected, but I don't want to manage multiple displays. Some servers I connect to (Citrix XenApps) have tantrums when they find multiple displays.
Take a look at the freestanding app to enable closed clamshell mode written by Misha Cargal, called ReClammer.app -- he posts the code for review, in case you worry about this or that. https://mbcargal.wordpress.com/2012/08/10/reclaiming-clamshell-mode-under-lion
After I ran this, my machine hung once at boot. I disconnected the external turned it off. Then, I booted the MBP, rapidly closing the lid, plugging in the cable to the external and powering up the external, all before the chime sounded. I opened the lid to select the FileVault2 user and enter the passphrase, hit return, closed the lid, and let the boot complete. I opened the lid -- recently a moment of frustration and hissed oaths -- and Voilà!
I hope ReClammer works for other frustrated people.