Everyone has different tolerances with this sort of thing. I have a 3D Vision setup, and even under the best circumstances, I get a little bit of nausea. I keep my playing sessions to 30-60 mins as a result.
As it turns out, even in 2D, I experience nausea at less than ~80 FPS. It is just a matter of how long it takes before it bothers me. The lower the FPS, the quicker I get nauseated. 3D Vision doesn't let me hit 80 FPS, so I have to deal with it. I feel the same at 60 FPS in 2D or 3D.
I'm pretty certain my problem is latency related, when it comes to something that moves so closely with my body, as a mouse does when controlling my view in a 1st person game. This is only a problem with a mouse in 1st person or over the shoulder viewed games. I believe it is related to the tactile feel of a mouse. Your view followed precisely with your hand movements, where as with a joystick, your view does not feel like an extension of your body. The Oculus Rift is likely to have the same effect on me as the mouse does, perhaps even more.
The question I do not know, is if the Oculus Rift would feel more real to me, and consequently make me feel sicker with even less latency than the mouse does, or perhaps it will be the same for me. While others may not get the connected feel with a mouse, that I do, they may get the same effect with the Oculus Rift.
As it stands, 3D Vision does not make my nausea problem worse. 60 FPS in 3D, effects me similarly to 60 FPS in 2D. It is the latency that bothers me, and it bothers me with a mouse. It may be the same with the Oculus Rift. While others never get that feeling with a mouse, the Oculus Rift may hit that threshold of realism, to make latency bother them.
Of course not everyone experiences motion sickness, and may not be bother by any of this.
As far as being sick goes, I do not recall any worse experiences with gaming in 2D or 3D. Or at least, the same tolerances exist, though I may quit a little sooner while sick.