One thing for sure, Doom 1 & 2 on the 360 BFG version is definitely not 60 fps. It is either 30 fps or 35 fps. It is smooth, but not 60 fps smooth.
Now, 3D is full frame packed in the 360 version, meaning you get 1280x720 per eye. I did not experience any of the issue Dark mentioned. After 30 minutes of gameplay I do not have any nausea feeling, eyestrain or headache. I left the default setting most of the time, but was able to pull it all the way to maximum without any trouble for the last 5 minutes or so. No cross eye feeling or anything of the sort. I am playing on a Playstation 3D display.
I like the 3D, but I am still uncertain whether I will play the game in 2D or 3D. Doom 3 definitely takes advantage of the 3D, with its tight corridors and all. You always feel it. On the other hand, 60fps is 60fps....
Doom 1 ran at 35 fps on PC yet it didn't look jarring as fuck on 60hz monitor...
No it's running Id Tech 4, but they took note of Tech 5 for some stuff. That's all according to Carmack during its keynote.