  Peculiar velocities of galaxies hosting Type Ia supernovae generate a
significant systematic effect in deriving the dark energy equation of state w,
at level of a few percent. Here we illustrate how the peculiar velocity effect
in SN Ia data can be turned from a 'systematic' into a probe of cosmological
parameters. We assume a flat Lambda-Cold Dark Matter model (w=-1) and use low
and high redshift SN Ia data to derive simultaneously three distinct estimates
of the matter density Omega_m which appear in the problem: from the geometry,
from the dynamics and from the shape of the matter power spectrum. We find that
each of the three Omega_m's agree with the canonical value Omega_m=0.25 to
within 1 sigma, for reasonably assumed fluctuation amplitude and Hubble
parameter. This is consistent with the standard cosmological scenario for both
the geometry and the growth of structure. For fixed Omega_m = 0.25 for all
three Omega_m's, we constrain gamma = 0.72 +/- 0.21 in the growth factor
Omega_m(z)^gamma, so we cannot currently distinguish between standard Einstein
gravity and predictions from some modified gravity models. Future surveys of
thousands of SN Ia, or inclusion of peculiar velocity data, could significantly
improve the above tests.
