Currently Being ModeratedOct 9, 2012 5:14 PM (in response to shmootz5)
Same problem here.
Apple, please _restore_ this fundamental functionality that is making my monitor useless 
That is a fu&ˆ@#ˆ*&@ shame!
I'm really tired of this problem. I can't use my external display and I can't use ANY external projector!
Currently Being ModeratedOct 9, 2012 5:53 PM (in response to shmootz5)
Has anyone updated with the 10.8.2 Supplemental from a couple nights ago? I updated, did a NVRAM reset, removed my boot-args flag and all seems to be good.
Haven't restarted since updating to the Supplemental, and have moved the 'Pro around between work and clamshell mode with no problems whatsoever.
Apple Support has also contacted me directly. If I get anything useful from their engineering team I'll pass it along as well.
If App Store isn't showing you the Supplemental you can always download it from the support site:
Currently Being ModeratedOct 10, 2012 2:19 AM (in response to yectep)
Same situation. I won't know for sure until a few days of solid usage (as the problem occured sporadically it seems), but early signs appear that it has solved the issue! I only updated today; I had no idea there was an update avaialable until you posted that message, so thank you!
And thank you everyone for your suggestions; I was waiting for Apple to solve it officially before playing around with my machine.
Currently Being ModeratedOct 11, 2012 2:26 AM (in response to yectep)
Well, it did it for me as well. After a couple of days testing it, it seems the latest update has fixed what the previous one had screwed up.
Thanks Apple for addressing the issue so swiftly!
Currently Being ModeratedOct 12, 2012 11:16 AM (in response to JBrey)
After 10.8.2 update, MBP (MBP8,2) no longer recognized external HP 2335 monitor. Same problem after installing OS 10.8.2 Supplemental. However, it recognizes a second external monitor, Acer S232HL. Curiously, my MBAir (3,1) (also w/ OS10.8.2) has no problem recognizing both the HP andthe Acer. Repeated side-by-side comparison several times (used same MiniDisplay-DVI adapter, both with reboot and while running): same result each time – MBP will not recognize the HP monitor (but sends signal to the Acer) while MBAir recognizes both. So for me, the problem appears to be tied to a specific computer (MBP8,2) and monitor (HP 2335) combination.
Currently Being ModeratedOct 13, 2012 10:47 AM (in response to JBrey)
OK. This does solve the external monitor resolution probel, but I still can't have my MBP lid open with disply off while having only the external monitor on. This is important for cooling purposes.
Currently Being ModeratedOct 13, 2012 11:47 AM (in response to veljkobb)
It's a good enough workaround I suppose. There's always the magnet trick where you can put a couple magnets on the lip of the MBP and will trick it into thinking your monitor's closed. Personally I prefer the screen trick since I sometimes do need that extra real estate, eg when I'm watching Netflix fullscreen.
Currently Being ModeratedOct 13, 2012 2:01 PM (in response to veljkobb)
Since you reset the nvram settings, run the following command again and reboot. I normally close the lid until i see the login screen on my external monitor and then i open the lid.
# sudo nvram boot-args="iog=0x0"
Thanks to Apple for fixing the external display issue quickly.
Currently Being ModeratedOct 19, 2012 10:41 AM (in response to yectep)
After a week of testing it seems that the problem is still there.
Here is what I have done:
1) reset nvram settings
2) # sudo nvram boot-args="iog=0x0"
Here is what the situation is:
If i press reset button and close the lid before the system is up, I'll get my external monitor on and everything will be fine (meaning, resolution on the external monitor is high) as long as the lid is closed. If I open the lid display will go on and the resolution on the external monitor goes down (which is my main issue). This would be OK or at least something that I can accept BUT, when I disconnect my external monitor and use lap top (so I have opened the lid and woke up the system) once I plug in the external monitor again, I have to restart the system in order to get external monitor on while the lid is closed.
Why this is an issue to me:
I have kind of old MBP (early 2008) and even my external monitor is of relatively low resolution graphic card can not provide high resolution on display and external monitor at same time (the highest I get is 1280x1024 having upper and portion of the screen black after reducing resolution on the display to the minimum one).
Now, why would I like to keep plugging and unplugging the external monitor all the time. Well, first this is something that I would like to be able to do, no matter why, and then the real reason is that I have only this one computer and I use it at the office as a desktop with external monitor and then I take it with me home where I use it as a laptop.
Basically as long as I'm not able to turn off the display and have external monitor on I have a problem. The minor issue is that I'd like to have the lid open while display off, but that is less important at this point.
I'm disappointed that Apple can't fix this for more than a month and a half already. Or at least I haven't found the solution on the web.
Currently Being ModeratedOct 19, 2012 11:08 AM (in response to veljkobb)
Well, same here. After a week of testing in the end the problem is not solved.
It worked in the first reboot but once I unplug the monitor and the laptop goes into sleep mode, the workaround doesn't work anymore.
This was something that Snow Leopard did without hacking NVRAM settings... can't understand why Apple changed this setting. Doesn't it make sense to choose the external monitor to be the only screen on while keeping the lip up for ventilation purposes?
Currently Being ModeratedOct 22, 2012 1:48 AM (in response to veljkobb)
Similar/(same) issue here! In Clamshell mode the monitor is still on, and my external monitor is showing very low resolution and won't change... great /sarcasm.
APPLE PLEASE FIX (PROPERLY THIS TIME)!
Edit: found a temporary fix (albeit a bit complicated). Set your displays as shown (I have 2 external monitors; one VGA and one USB. The internal monitor is on the bottom right). Also, you will then need to manually change the resolution of the external display.
2 problems with this: 1) It must be done every time the computer boots up and 2) the internal monitor is still on.
Currently Being ModeratedNov 21, 2012 12:40 PM (in response to shmootz5)
SOLUTION TO HAVING LID OPEN BUT DISPLAY OFF
I'm not sure if this is entirely the problem you're experiencing, but... Until I updated to 10.8.2 (from Snow Leopard) I had no problems with Clamshell mode, I could work in an external display with the MBP's display open and OFF providing that I kept the lid closed during boot. Since 10.8.2 this stopped working. As soon as I open the lid, the MBP's display switches on. So, not wanting to revert to using magnets(!) and such I thought I'd try Disk Utility and that did the job:
1. Open Disk Utility.
2. Click on your HD's volume.
3. Click "Repair Disk Permissions".
4. Once complete (and you probably had a load of errors) Run #3 again. This time you should have no errors.
5. Turn off your Mac.
6. Turn on your Mac with your external monitor switched on.
7. When the "Laaaaaa" sound is heard, close the lid on your MBP.
8. Log in.
9. Open the lid of your MBP.]
10. You should still be in clamshell mode and your MBP's screen should still be off.
I've tried this twice on mine before posting this and it seems to have fixed that problem.
Currently Being ModeratedNov 25, 2012 7:18 AM (in response to shmootz5)
Ah. I think I've found the/a problem. The solution I posted worked fine until I encrypted my HD with FileVault. After that there is no way it will run in clamshell mode with the screen open for cooling. In fact, it won't even startup in clamshell mode - the screen, keyboard and magic trackpad don't even turn on until AFTER I log in on the MBP.
I don't know if any of you are using an encrypted disc but I'm decrypting mine to see if it resolves the issue and will report back.
Currently Being ModeratedNov 26, 2012 6:22 AM (in response to shmootz5)
OK, this is the straw that breaks this camel's back:
I have been extremely dissatisfied with Lion and now Mountain Lion.
I use an Apple 2560x1600 (yes, not 2560x1449) cinema display as my main screen. The laptop sits behind it on a shelf, with its lid closed and its screen off. This worked superbly well up to Snow Leopard.
It stopped working in Lion, but I did find the workaround sudo nvram boot-args="iog=0x0"
However in 10.8.2 this does not work. Setting the kernel in 64bit mode does not work either.
If I leave the laptop's screen on (i.e. lid open) then I tend to lose the mouse pointer on the other screen, there is light I do not want, and so on.
Mountain Lion is uncomfortable for content creators, and a simple nuisance compared to Snow Leopard.
In September 2011 (not 2012!) I bought a second laptop just for the purpose of testing Lion while still being productive on my Snow Leopard laptop. In over a year of struggling I have not succeeded in switching yet.
I am so fed up that I will now just switch to Ubuntu Linux.