It is possible to get dynamic hair to react to collisions. There are three areas that have to be enabled. You must turn on Collision Detection for the Document window, you must go to each body part and turn on collision detection for the body part, and then you must select the collision detection for the hair. Then you must press the Calculate Dynamics button. But after having done all that, you must be aware that collision detection for hair does not work yet in Poser5. It is full of bugs, and your results tend to have hair jumping every which way.