The physician's right of refusal: What are the limits?

A physician's long-established right to refuse to provide a requested service based on his or her moral beliefs is being challenged. Some authors suggest that physicians should not be licensed if they are unwilling to provide all legal services. Others would grant them the right to refuse, but require them to refer to a willing professional. What are the limits of a physician's right to refuse? When such a right is claimed on moral grounds, what residual obligations does the physician have to the patient? How should the profession (or society) decide when a moral claim to a right to refuse is justified?