What kinds of things are they doing in the PT sessions?

modalities to ease the pain and relax the muscles, (such as heat, massage, triggerpoint work, ultrasound, e stim, LLLT?)
- or having you try to strengthen the muscles when they are still painful.

I usually suggest expert chiropractic or at least some evaluations by good ones. maybe even upper cervical ( atlas/axis or c1,c2 care)
for info on it see - signs & symptoms - http://www.upcspine.com/self.htm
www.upcspine.com

With a whiplash type injury there is also some chance of later on acquiring thoracic outlet syndrome (TOS) especially if you have a repetitive or desk type job or hobby.
much more info on TOS- http://neurotalk.psychcentral.com/thread84.html