This is speculative, but it seems to me like 99% of the motion sickness that happens in the Rift is because there's only rotation tracking--no motion tracking.

When we look around, we move our head subtly side to side and forward and back. The Rift dev kit doesn't do that--it just tracks what direction you're pointed in. My feeling is that causes a big chunk of the motion sickness.

The production unit will include motion tracking. As long as game titles take advantage of it, and don't do stuff like shake the camera, or move it too much, or do too many cuts, then I think 99% of people will be fine. For those titles at least. Other titles will push the limits.