Well, at least the press event for the opening of M:S is still set for October 9, right?
As for why some people experience motion sickness, it may be that some become disoriented by the video, but I would attribute it more to the centrifuge system itself. You could turn off the video, and you would still find some people feeling ill.
I speak from experience working with centrifuges that do not have video systems, i.e.:
http://www.spaceadventures.com/terrestrial/centrifuge/index.html
It is true that on M:S, if you follow the instructions given (face forward, attention on the video and keep your eyes open) then you shouldn't be aware of the spinning. Those with more sensitive inner ear systems though, may still be affected.
That said, for the majority, I would guess that one of three situations are at play:
1. Closing your eyes or not focusing on the video - without the benefit of visual input, there is nothing to correct for the forces being registered by your inner ear. Given that on other ride systems that Disney guests are accustomed to (Star Tours, Body Wars), closing ones eyes may help alleviate dizziness, even given the warnings, some may fall back to the familiar. Even those who do not close their eyes but instead focus elsewhere in the capsule (rather than on the video) may be causing some to feel ill.
2. On that note, even with your eyes open, for some, moving your head around rapidly will cause the g-forces to produce varied movement of the hairs suspended in liquid in your inner ear, a.k.a. dizziness.
3. When I rode M:S, I was surprised by how many warnings of possible sickness are provided -- and I would suspect (though I have no proof of this other than what I have seen with Space Adventures' clients) that some of these warnings are creating a self-fulfilling prophesy. On my first ride, even though I knew what to expect, by the end of the pre-show I was experiencing mild butterflies. For someone with no previous centrifuge experience, I could imagine the psychological effects being much greater.
So, for the original poster writing for the guide, my best suggestion would be to be honest about the sensations experienced and explain to keep your eyes open and face forward, but I wouldn't spend too much time on the potential for motion sickness (and might even advise to stay calm during the pre-show story building).