There is an article on Polygon.com about the Oculus Rift people working to solve reverse motion sickness. Polygon claims this is THE major obstacle for the Rift. I am certain it is a problem. I certainly am hearing more people mention motion sickness in reviews of the Rift. Whether it is THE problem or not I am less sure.
As the article points out, motion sickness is about our sense detecting motion that our eyes cannot see. This is like being below deck on a rolling ship. You can feel the roll. But, the room appears stationary.
With the RIFT the problem is the reverse. Your eyes are seeing motion that your senses do not detect. The disconnect between the visual sight and other senses is thought to be the cause of motion sickness. But, the cause of motion/simulator sickness is not settled science.
The medical community spends little effort or time on solving the problem of motion sickness. In general it is not life threatening. So, there are higher priority medical issues to concentrate on.
There is also a genetic component. Japanese people are much more prone to motion sickness, which has never made sense to me. How does a seafaring people not adapt a coping mechanism? Whatever, it may be geneticists that finally cure this problem.
In the case of the Rift it is not truly motion sickness. So, a newer name is used: Simulator Sickness. For now they are the ones leading research on simulator sickness.