This is not so much a question as an observation.
I was using a 1920 x 1200 Cinema HD Display 23 inch (LCD) with my iMac as a second monitor. I needed to move it temporarily to a Mini. I unplugged it hot from the iMac and hooked it up to the Mini.
System Profiler and Displays in System Prefs showed it as active and working. But the little blue LED on the front was off and the screen was dead.
After making sure it was not lint, unplugging and plugging back in, restarting, etc. I tool the display back over to the iMac.
Same with it. Less than 30 minutes earlier, it had worked for weeks fine on the iMac.
I did searches here and on the web to see it this was a common problem. I did see things about bad power supplies and similar (although not exact) HD displays. I also kept seeing expensive repairs.
Because I was pooped from working in the yard over the weekend. I decided to unplug it, shut off the iMac, and try in the morning.
The next morning, no problems at all. Whatever it was, seemed to reset. The display is working fine again on the iMac.
So if you have a Cinema HD Display go black on you but still shows up in System Profiler and Displays in System Prefs as on, don't panic. Go have lunch. Go home. It might just come back on after being unplugged for some time.
I guess this is the question: What happened?
BTW: it had previously been used for years on the Mini before being needed on the iMac. I hooked another non Apple monitor up to the Mini and it worked fine. So, it was not a Mini issue and, also, it failed to come on when brought back to the iMac, at first.
Something reset in the Cinema HD Display when left unplugged.