I try to explain as best as I can.
A DLP projector throw the image to the wall via a rotating colour-wheel, which sends via a special mirror inside the projector. This happens in a certain frequancy. Some people (but not all), can actually see the image flickering because of this. Then it is very easy to get car-sick. Because your eyes will focus on the flickering at the same time as focusing on the projected image.
So If you go for DLP, be sure that you test it carefully first. Maybe someone else here can explain this phenomenon better than my norwenglish language;)