CCPhysics objects create Objective-Chipmunk objects which in turn create Chipmunk objects. Chipmunk and Objective-Chipmunk provide "user data" properties so that you can find out what object owns something.
When CCPhysics creates an Objective-Chipmunk object, it sets the user data pointer to the CCPhysics object. When Objective-Chipmunk creates a Chipmunk object, it sets the user data pointer to the Objective-Chipmunk object. So you can just follow these pointers back. You can use
CCPhysicsShape.node to find out what CCNode they are associated with.
For example, with bodies:
cpBody *body_C = ...;
ChipmunkBody *body_objC = cpBodyGetUserData(body_C);
CCPhysicsBody *body_CC = body_objC.userData;
CCNode *node = body_CC.node;
Since you are working with Objective-Chipmunk objects, you can skip the first two lines, but I thought I would include them for completeness.