Clamshell Mode Doesn't Work after Upgrade to 10.8.2

Hi, I have the same problem with an MBA 11´. I realized the problem arises when the mac is not connected to the electric supply. Once connected it to the power supply and it seems to be working fine now. I also have an MBA 13, and it doesn´t have this problem.

I tried "sudo nvram -d boot-args" and reboot and it still was shutting off. Then I plugged in Macbook Air to the external power source and I can now work in clamshell mode without the Mac going to sleep. Apparently the external power needs to be connected for the clamshell mode to work.

THANK YOU for sharing this dead simple fact !!!!!!

I had rewired all my (music) equipment, unplugging everything in the process. I use a dock (Henge Docks, highly recommendable by the way) to avoid plugging in each time I need them all the peripherals I need for my music.

This dock obviously includes the power connection. When I replugged everything, I forgot (yeah... I know) my MacBook's power supply, and didn't notice because the battery was full and everything else was working.

It made no sense that suddenly my laptop wouldn't wake up, clamshell had worked less than 12 hours earlier, I was loosing my mind.

I hope my thanks reach you. You helped me avoiding all those surely useful but very scary terminal commands (scary to the very basic user I am.)

Not trying them was a very good thing as I never use clamshell on battery, so my hilarious "problem" basically solved by plugging in what had always been plugged in...