You guys mean the vsync cable right? I just ran into the same issue on a mid 2011 27". Inverter board was faulty, so i've changed it, now if i turn the brightness control over 5, it starts to black out. The only way to wake it is the ctrl+shift+eject then a mouse click, but then it does it again.
I have to mention though, that the broken inverter board is the Sumida 612-0094 and i could only replace it with the V267-604HF...dunno if that could be a problem, because it's used inside the same iMac with probably the same LCD display...
If the problem can be the board, please tell me, I can return it today or in a few days if it's not ok!