  We perform a maximum likelihood analysis of the cluster abundance measured in
the SDSS using the maxBCG cluster finding algorithm. Our analysis is aimed at
constraining the power spectrum normalization $\sigma_8$, and assumes flat
cosmologies with a scale invariant spectrum, massless neutrinos, and CMB and
supernova priors Omega_m*h^2=0.128+/-0.01 and h=0.72+/-0.05 respectively.
Following the method described in the companion paper Rozo et al. 2007, we
derive \sigma_8=0.92+/-0.10$ (1-sigma) after marginalizing over all major
systematic uncertainties. We place strong lower limits on the normalization,
sigma_8>0.76 (95% CL) (>0.68 at 99% CL). We also find that our analysis favors
relatively low values for the slope of the Halo Occupation Distribution (HOD),
alpha=0.83+/-0.06. The uncertainties of these determinations will substantially
improve upon completion of an ongoing campaign to estimate dynamical, weak
lensing, and X-ray cluster masses in the SDSS maxBCG cluster sample.
