The Cat Whisperer: The Secret of How to Talk to Your Cat by Claire Bessant:
The cat's sense of balance and its ability to fall on its feet have an almost supernatural reputation... This special sense of balance, which is based in the inner ear, may be the reason that cats do not suffer from motion sickness, a feeling of nausea which affects both humans and dogs when the head had frequently to change its position relative to the body, such as riding in a car or sailing in a boat. While cats may not enjoy traveling in the car, they are no usually sick, unlike puppies or young children who seem to be affected more than their adult counterparts (pg 6).
Cat Owner's Home Veterinary Handbook by Delbert Carlson, DVM and James Giffin, MD:
Many cats suffer from car sickness and may become seasick when traveling by boat or air. The signs are restlessness followed by salivation, yawning, nausea and then vomiting. Motion sickness is caused by overstimulation of the labyrinth system in the inner ear. Most cats will get over it once they adjust to traveling. If you suspect your cat is going to be sick, a veterinarian may suggest that you give Dramamine, 12.5 mg for the aver-sized cat, one hour before traveling. Withhold food before taking a trip. Cats travel best on an empty stomach (pg 189).I have not experienced a motion sick cat personally. Whenever I travel with any of my cats, they have not enjoyed the ride very much, but they haven't gotten motion sick. So, even without the personal experience, I am more inclined to believe that which was written by licensed veterinarians - cats are susceptible to motion sickness.