Whiplash injury usually occurs with rear-end impacts. The neck is forced into flexion and then into extension. The sudden movements take place in a matter of seconds. Many studies have been done using live subjects, crash dummies, and computer simulation to define the steps of events that occur with whiplash injury.
The results are not very clear. Studies in the U.S., Canada, and Australia have shown both results: increased cases of chronic neck and back pain with and without the use of seatbelts.
The main conclusion is that seat belts save many lives each year. The life-saving ability of seatbelts far outweighs the risk of increased injury during a rear-end collision. The bottom line is: you should always wear your seatbelt.