As reported last week in Science, a team of researchers in Cambridge have demonstrated that a patient in vegetative state preserved conscious awareness. Using a fMRI scanner the patient showed same activation patterns as healthy volunteers when she was asked to imagine playing tennis. Dr. Adrian Owen, the leader of this research, claim that the vegetative diagnosed brain was able to understand the meaning of sentences and respond consciously. For a detailed description of the research visit the Medical Research Council website. This specific research work is published in Science, 8th September 2006 under the title Detecting Awareness in the Vegetative State.
If these research conclusions are confirmed, it means that current techniques to assess the level of consciousness of humans are not fully valid. One might be unable to move or speak, however that does not neccesarily means that the subject is unable to experience some level of consciousness.
Taking this idea about consciousness level assesment to the field of machine consciousness, one could think about the best way to determine the level of awareness of a robot. Obviously, the first reference is always the Turing test. However, as demonstrated in humans, a purely external evaluation could not be valid in terms of assessing the real level of consciousness of an artificial entity. Anyway, from a strictly engineering point of view this question is irrelevant.