Using a mid-infrared calibration of the Cepheid distance scale based on recent observations at 3.6 mu m with the Spitzer Space Telescope, we have obtained a new, high-accuracy calibration of the Hubble constant. In combination with the new Spitzer calibration, the systematic uncertainty in Hsub 0 over that obtained by the Hubble Space Telescope Key Project has decreased by over a factor of three. The result, in combination with WMAP7 measurements of the cosmic microwave background anisotropies and assuming a flat universe, yields a value of the equation of state for dark energy, wsub 0 = -1.09 + or - 0.10. Alternatively, relaxing the constraints on flatness and the numbers of relativistic species, and combining our results with those of WMAP7, Type Ia supernovae and baryon acoustic oscillations yield wsub 0 = -1.08 + or - 0.10 and a value of Nsubeff = 4.13 + or - 0.67, mildly consistent with the existence of a fourth neutrino species.