Like SpikeX says, rigging should be done in the modelling software, once you've done that you add rigid bodies and joints to the bones of the hair model much like regular rag dolls. If you were using a non-physics simulated main character model but with ragdoll hair you simply set the body to kinematic and put a fixed joint on it to make the hair stay on the character but still be able to rag doll.
answered May 02 '10 at 09:52 AM
I know this is way late, but a possible solution for you is to instead use the skinned cloth renderer: http://docs.unity3d.com/Documentation/Components/class-SkinnedCloth.html
It won't interact with the environment, but it will give you flowing beautiful hair on your characters :)
answered Mar 30 at 08:50 PM