This problem just cropped up for me.
Early 2008 Mac Pro Quad Core
10 Gb RAM
Nvidia GeForce 8800 GT 512 mb
OS X 10.8.1
Cinema HD Display 30
After either putting the Mac to sleep, or letting it go to sleep itself, the primary display does not re-awaken and stays black. The light in the lower right of the monitor sometimes turns on and off. The secondary display turns on and shows what should be there.
I followed some advice found in this forum and unplugged the mouse and keyboard usb plugs from the back of the monitor. The monitor came to life. I plugged the mouse and keyboard into usb ports connected to a usb hub connected to the front of the Mac Pro desktop enclosure.
While able to use the primary monitor I followed the suggestion to go to System Preferences, Energy Saver and change Computer sleep to Never. I changed Display sleep to 3 hrs because I did not want to change it to Never.
This results in the monitor now showing normal, but it randomly blanks out and then returns. Sometimes for 1 second and sometimes for 5-10 secs.
I then replaced the apple mouse with a Kensington mouse. For the time being, this seems to have solved the problem. It is not a final solution until I can test the computer in different scenarios.
If this does prove to solve the problem, I would appreciate hearing from an Apple Rep telling us why an apparent faulty Apple mouse causes the monitor to blank out both permanently or randomly.
It is easy enough to buy a new mouse but it is important to understand why this happened.