One thing that helps with Soarin' as opposed to IMAX is that you actually are moving in synch with the image on the screen - a lot of motion sickness is caused by a disconnect between what you are seeing and the actual motion you are having. For example, I do get motion sickness when watching some of those 3-D movies that simulate a roller coaster, but do not get motion sickness on an actual roller coaster. That's why they always tell sea sick people to go up on deck and look at the ocean - if you are rolling around in your cabin, but the visual perception is that nothing is moving (because your cabin walls and furniture all appear to be stationary), that causes the disconnect. It's why some people can't read in the car (the insides of the car are all fixed relative to each other no matter how fast you are actually going) but feel fine looking out the window in a car.
Of course, I get that some people are more sensitive than others, so your mileage may vary.