I think that this is such a fluid and difficult concept to grasp in social work because it is such an empathic and giving and vulnerable space. I can imagine that the boundaries are not entirely clearly defined, and perhaps must be put forth by the individuals involved--and each individual may have different boundaries they are willing to set.
First and foremost, because social work is such vulnerable work, I believe that relationships built on respect and trust is paramount. It's also essential that the person "in power" keeps themselves in check--I have found in more than one place of work that the manager or person in power has had a tendency to push the envelope and perhaps move beyond professional boundaries just because they can, or they let emotions get in the way of their rational thinking.
Obviously there is so much more that I will be learning as I move through this process but those are the two initial thoughts I have on the subject.