I was having the same issue, running an AMD phenom X4, 16GB Corsair DDR2 800Mhz ram, with 2x Geforce 9600 GSOs in SLi. EDIT: On Windows 7 RTM.
It obviously is not a system problem for me, nor is it a power problem, since I'm using 2x 800 watt PSUs, one of which is dedicated to the graphics cards.
I was only getting the issue when I was playing long games, and it would only happen after about 25 to 30 minutes.
I obviously had everything on max settings, but I remembered hearing somewhere that some shadow rendering can cause the issue.
I turned shadows down to low and played a game against 3 hard enemies. Took me about 45 minutes to finally destroy the citadel, but the game never blacked out once.
To anyone else with this problem are your shadows turned up to high? If so, turn them to low and post your results.