I have the issue with 'Black Screen' on my late 2010 27-inch iMac. Boots up fine, but then randomly goes black. The iMac is still alive as I can still hear things playing. Someone suggested I try putting the iMac to sleep using the cmd/alt/eject key commends. Well I tried this, and after doing the key commends then touching any key afterwards the iMac came back. Screen came back on. It did however go off again after a couple of minutes, but used the same tactics again and it worked again. Still a pain, but a solution.