In my spare time (cough cough) I will sometimes listen to lectures that I find on iTunes University to educate myself on a range of topics that I might otherwise not have time, nor the access to study.  If you get the opportunity I highly suggest going to iTunes University and searching some of the courses and lectures that they have to offer. 

Currently, I am listening to Jeremy Wolfe at MIT who is an excellent speaker.  In a recent lecture he discussed the causes of motion sickness (which correlated with other work I was studying at the time.)

The brain has a number off areas purely used for integration information from a variety of sensory systems.  These association areas can be in a variety of places in the brain, including the cortex and several midbrain/hindbrain structures. 

Now, in the case of motion sickness what happens to the brain during that bumpy airplane ride that we feel ill?  Well, the vestibular system which senses balance is constantly trying to correct itself as it is being thrown about the plane.  In another sense our visual system is also allowing for certain self-corrections using the vestibular-occular  reflex.  This reflex can be demonstrated by staring at this word and moving your head back and forth.  The VOR is a very robust system that allows images to stay clear on your retina using information between the visual and vestibular system in an open loop.

Most often when flying in a plane you do not fully register to bumps, motions, and minor jolts of air pressure changes over the wings.  What part of your body does notice?  The vestibular system.  Your brain now is stressed with the task of trying to processing visual data that is held constant by the VOR and a highly active vestibular system.  Because each sensory system is providing a different interpretation of the world the brain triggers a line of defense including nausea.  This system is based on a symptom brought on by evolutionary equation.

Equation:

If the brain cannot integrate sensory systems then the body has been poisoned by a noxious neurotoxin.  Therefore nausea triggers the body into removing whatever the body has most recently ingested because it may have been poisoned by a neurotoxin (i.e certain species of frog, mushrooms, berries, etc.)