  On-going projects to discover Type Ia supernovae at redshifts z = 0.3 - 1,
coupled with improved techniques to narrow the dispersion in SN Ia peak
magnitudes, have renewed the prospects for determining the cosmic deceleration
parameter q_0. We estimate the expected uncertainty in the Hubble diagram
determination of q_0 due to weak lensing by structure in the universe, which
stochastically shifts the apparent brightness of distant standard candles.
Although the results are sensitive to the density power spectrum on small
scales, the induced flux dispersion sigma_m < 0.04 Omega_{m,0}^{1/2} mag for
sources at z \leq 0.5, well below the ``intrinsic" spread of nearby SN Ia
magnitudes, sigma_M \simeq 0.2 mag. Thus, density inhomogeneities do not
significantly impact the current program to measure q_0, in contrast to a
recent claim. If, however, light-curve shape and other calibrators can reduce
the effective intrinsic spread to 0.1 mag at high z, then weak lensing could
increase the observed spread by 30% in an Omega_{m,0}=1 universe for SNe at z >
1.
