Black hole entropy from horizon conformal field theory

String theory and "quantum geometry" have recently offered independent statistical mechanical explanations of black hole thermodynamics. But these successes raise a new problem: why should models with such different microscopic degrees of freedom yield identical results? I propose that the asymptotic behavior of the density of states at a black hole horizon may be determined by an underlying symmetry inherited from classical general relativity, independent of the details of quantum gravity. I offer evidence that a two-dimensional conformal symmetry at the horizon, with a classical central extension, may provide the needed behavior.