I think the mirror test demonstrates, at the point where its popularity as a measure of self-consciousness intersects with its inadequacy, the tendency of scientific investigation to wander into tautology when it treats the phenomena of sentience. It requires great care and imagination to conceive an experiment that will yield some verifiable external measure of an internal process, and when someone succeeds as elegantly as the originator of the mirror test, there's a strong temptation among those who credit the significance of the results to "move forward," to avoid any needless backtracking (e.g. to the definitional boundaries of the phenomenon under scrutiny).
Thus the question of whether an animal possesses self-awareness elides irresistibly with the question of whether he can, with the help of a well-placed polka dot, make a connection between his kinesthetic or proprioceptive sense and an alien image that (bizarrely) coordinates with it; in the absence of any similarly compelling measure, the mirror test becomes definitive for hundreds of scientists who go on to paint scores of unsuspecting animals in their sleep. Will a parrot pass or fail? A tamarin? A zebra? As Frans de Waal observes, "for better or worse, this test has remained the gold standard of self-identity."
Even the test's critics seem to accept its foundational terms: if, they say, an orangutan who touches a spot on his forehead really understood the image in the mirror as a representation of his own body, then the conditions for self-awareness would be met. But, they argue, he probably just likes to poke at his face. Or he learns to do so because it makes humans grimace in that weird way that means more dates and sunflower seeds.
Again, methodological limitations lead us to chase our tails: the mirror test measures the capacity for self-consciousness because... we don't have a better test. Or a more complete one. Hell, I don't know what it means to be self-conscious! Do you?
My feeling, one I'd like to develop into a well-reasoned conviction (so goes the trajectory of my mental life), is that there ought to be a kind of intellectual affirmative action in the direction of granting non-human animals manifold intelligence and complex consciousness. We ought to assume they're endowed with great riches of thought and feeling until they prove otherwise, though we ought not to assume that their thoughts and feelings trace the same patterns as ours. I have some sympathy for Marc Hauser, despite his faults and all the damage he's done to the cause of anthropomorphology, because a bias in favor of non-human intelligence remains so rare, while the bias against almost defines "respectable" research.