  Using a mid-infrared calibration of the Cepheid distance scale based on
recent observations at 3.6 um with the Spitzer Space Telescope, we have
obtained a new, high-accuracy calibration of the Hubble constant. We have
established the mid-IR zero point of the Leavitt Law (the Cepheid
Period-Luminosity relation) using time-averaged 3.6 um data for ten
high-metallicity, Milky Way Cepheids having independently-measured
trigonometric parallaxes. We have adopted the slope of the PL relation using
time-averaged 3.6 um data for 80 long-period Large Magellanic Cloud (LMC)
Cepheids falling in the period range 0.8 < log(P) < 1.8. We find a new
reddening-corrected distance to the LMC of 18.477 +/- 0.033 (systematic) mag.
We re-examine the systematic uncertainties in H0, also taking into account new
data over the past decade. In combination with the new Spitzer calibration, the
systematic uncertainty in H0 over that obtained by the Hubble Space Telescope
(HST) Key Project has decreased by over a factor of three. Applying the Spitzer
calibration to the Key Project sample, we find a value of H0 = 74.3 with a
systematic uncertainty of +/-2.1 (systematic) km/s/Mpc, corresponding to a 2.8%
systematic uncertainty in the Hubble constant. This 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, w0 =
-1.09 +/- 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 yields w0 = -1.08 +/- 0.10
and a value of N_eff = 4.13 +/- 0.67, mildly consistent with the existence of a
fourth neutrino species.
