  We combine the CfA3 supernova Type Ia (SN Ia) sample with samples from the
literature to calculate improved constraints on the dark energy equation of
state parameter, w. The CfA3 sample is added to the Union set of Kowalski et
al. (2008) to form the Constitution set and, combined with a BAO prior,
produces 1+w=0.013 +0.066/-0.068 (0.11 syst), consistent with the cosmological
constant. The CfA3 addition makes the cosmologically-useful sample of nearby SN
Ia between 2.6 and 2.9 times larger than before, reducing the statistical
uncertainty to the point where systematics play the largest role. We use four
light curve fitters to test for systematic differences: SALT, SALT2, MLCS2k2
(R_V=3.1), and MLCS2k2 (R_V=1.7). SALT produces high-redshift Hubble residuals
with systematic trends versus color and larger scatter than MLCS2k2. MLCS2k2
overestimates the intrinsic luminosity of SN Ia with 0.7 < Delta < 1.2. MLCS2k2
with R_V=3.1 overestimates host-galaxy extinction while R_V=1.7 does not. Our
investigation is consistent with no Hubble bubble. We also find that, after
light-curve correction, SN Ia in Scd/Sd/Irr hosts are intrinsically fainter
than those in E/S0 hosts by 2 sigma, suggesting that they may come from
different populations. We also find that SN Ia in Scd/Sd/Irr hosts have low
scatter (0.1 mag) and reddening. Current systematic errors can be reduced by
improving SN Ia photometric accuracy, by including the CfA3 sample to retrain
light-curve fitters, by combining optical SN Ia photometry with near-infrared
photometry to understand host-galaxy extinction, and by determining if
different environments give rise to different intrinsic SN Ia luminosity after
correction for light-curve shape and color.
