  We examine the dependence of the mass-to-light (M/L) ratio of large-scale
structure on cosmological parameters, in models that are constrained to match
observations of the projected galaxy correlation function w(rp). For a sequence
of cosmological models with a fixed P(k) shape and increasing normalization
\sig8, we find parameters of the galaxy halo occupation distribution (HOD) that
reproduce SDSS measurements of w(rp) as a function of luminosity. Using these
HOD models we calculate mean M/L ratios as a function of halo mass and populate
halos of N-body simulations to compute M/L in larger scale environments,
including cluster infall regions. For all cosmological models, the M/L ratio in
high mass halos or high density regions is approximately independent of halo
mass or smoothing scale. However, the "plateau" value of M/L depends on \sig8
as well as \Omega_m, and it represents the universal mass-to-light ratio <M/L>
only for models in which the galaxy correlation function is approximately
unbiased, i.e., with \sig8 ~ \sig8_gal. Our results for cluster mass halos
follow the trend M/L = 577(\Omega_m/0.3)(\sig8/0.9)^{1.7} h Msun/Lsun. Combined
with Carlberg et al.'s (1996) mean M/L ratio of CNOC galaxy clusters, this
relation implies (\sig8/0.9)(\Omega_m/0.3)^{0.6} = 0.75 +/- 0.06. M/L ratios of
clusters from the SDSS and CAIRNS surveys yield similar results. This
constraint is inconsistent with parameter values \Omega_m ~ 0.3, \sig8 ~ 0.9
favored by recent joint analyses of CMB measurements and other large-scale
structure data. We discuss possible resolutions, none of which seems entirely
satisfactory. Appendices present an improved formula for halo bias factors and
an improved analytic technique for calculating the galaxy correlation function
from a given cosmological model and HOD. (Abridged)
