  We present initial results from a time-series BVI survey of two fields in NGC
4258 using the Advanced Camera for Surveys onboard the Hubble Space Telescope.
This galaxy was selected because of its accurate maser-based distance, which is
anticipated to have a total uncertainty of ~3%. The goal of the HST
observations is to provide an absolute calibration of the Cepheid Distance
Scale and to measure its dependence on chemical abundance (the so-called
"metallicity effect").
  We carried out observations of two fields at different galactocentric
distances with a mean abundance difference of 0.5 dex. We discovered a total of
281 Cepheids with periods ranging from 4 to 45 days (the duration of our
observing window). We determine a Cepheid distance modulus for NGC 4258
(relative to the LMC) of 10.88 +- 0.04 (random) +- 0.05 (systematic) mag. Given
the published maser distance to the galaxy, this implies \mu (LMC)=18.41 +-
0.10 (r) +- 0.13 (s) mag or D(LMC)= 48.1 +- 2.3 (r) +- 2.9 (s) kpc. We measure
a metallicity effect of \gamma=-0.29 +- 0.09 (r) +- 0.05 (s) mag/dex. We see no
evidence for a variation in the slope of the Period-Luminosity relation as a
function of abundance.
  We estimate a Hubble Constant of H_0= 74 +- 3 (r) +- 6 (s) km/s Mpc using a
recent sample of 4 well-observed type Ia SNe and our new calibration of the
Cepheid Distance Scale. It may soon be possible to measure the value of H_0
with a total uncertainty of 5%, with consequent improvement in the
determination of the equation of state of dark energy.
