One Pot Meal has some interesting reflections on professional authority, but I'm not sure it is so simple any more. Summarizing the sociological definition of a profession, I tell my students that a profession (as opposed to other kinds of jobs) has three characteristics:
1. its members have specialized knowledge
2. they are certified in some way (usually by other members of the profession--this kind of self-definition is called gatekeeping)
3. they have a responsibility to serve the public good.
When I talk about ethics I add that you get certain privileges as a member of a particular profession and in return you are required to follow the rules of that profession, you don't have the right to ignore those rules that you don't agree with (besides which, people who think they are above the rules usually end up deceiving themselves into serious trouble).
What strikes me whenever I teach these ideas is that we don't really believe any more that professionals are that honorable. The social contract by which we gave professionals authority is breaking down. We also have so much more access to information that we don't rely as much any more on the professional's specialized knowledge.