1. A partially implantable vestibular prosthesis system comprising: one or more external movement sensors attachable to the skin of a patient's head and configured to generate an external movement signal representing movement of the patient's head; an external transmitter in communication with the external movement sensor configured for transcutaneous transmission of an implant communication signal having an electrical power component and a signal component based on the external movement signal; one or more internal movement sensors implantable under the skin of the patient's head configured to generate an internal movement signal representing movement of the patient's head; a fail-safe sensor configured to detect movement of the one or more external movement sensors relative to the head and generate a corresponding relative motion signal; and an implant processor implantable under the skin and in communication with the fail-safe sensor, the internal movement sensor, and the external transmitter and configured to generate an implant stimulation signal based on one of the movement signals and/or the relative motion signal to electrically stimulate target neural tissue for vestibular sensation by the patient.