IIRC a SGI study pointed out that an untrained eye can see the difference in fps even above 120 fps.
Like Nutty I'm worried about the sync with the game frequency and the screen frequency. How syncronize 60 fps at 85 Hz vert refresh?
Fixing the game engine at 60 Hz means the CPU must always be able to do the math in time. I wonder how they manage to do this with exact timings.
Now, even if they got it right, that the whole game runs internally at 60 Hz, what's keeping them from interpolating the results when rendering each frames? IMHO it would be better 'cause you could get any fps you would want this way with a different frame each time.
For the moment this comment from JC makes me think Doom III will always feel jerky no matter the PC you've got.