  Recent papers have shown that a small systematic redshift shift ($\Delta
z\sim 10^{-5}$) in measurements of type Ia supernovae can cause a significant
bias ($\sim$1\%) in the recovery of cosmological parameters. Such a redshift
shift could be caused, for example, by a gravitational redshift due to the
density of our local environment. The sensitivity of supernova data to redshift
shifts means supernovae make excellent probes of inhomogeneities. We therefore
invert the analysis, and try to diagnose the nature of our local gravitational
environment by fitting for $\Delta z$ as an extra free parameter alongside the
usual cosmological parameters.
