I worked in an independent birthing center, and part of the appeal (for both the clients and the staff) is that there is none of the "it is unprofessional to become involved with your patients" kind of thing. Many of my closest friends are people whose birth I attended, or whose childbirth classes I taught. Of course, I didn't become best friends with everyone. And we all remained professional in terms of our roles while in the clinical setting. There are still professional boundaries, and for example, I do not give my phone number out to my clients--I do give my email, though, so that they can ask questions about classes. At the hosptial I do not develop these relationships with my patients. The setting is just so different, not conducive to developing personal relationships for many different reasons.
I think it is nice to have my practitioners, especially the ones I see over and over, and with whom I have experienced a very intense experience, to become more than just my "nurse" or my "doctor." I trust them more. Our pts at the birth center always comment on how much more respected they feel, how much they trust us, and how they feel that we are honestly concerned with their wellbeing. We get comments all the time to the effect of "the hospital nurses are so cold," or "it seems like it's just a job for the hospital docs." The hospital setting is just different, designed to serve large numbers of people in an efficient (well, relatively) manner. It's not designed for close, personal care. Birth centers, LTC, and hospice are some settings that I can think of where the rules or definitions of "professional boundaries" are different than the acute care setting at the hospital.
It's a gray line. I think if it is a power thing, or feels wrong to anyone involved, then it probably is over the line. But different strokes for different folks, you know?