From a Penny Arcade Report interview with id software on Doom 3: BFG Edition:
the game runs at 60 frames per second, even in the 3D? "Well, mostly," Tim Willits, creative director at id Software, admitted. So how the hell do you make a game look this good, on the 360, in 3D, at such a high frame rate? "We have John Carmack," he explained.