Erin Sheldon gave an absolutely wonderful astro seminar today about his work on weak lensing of the MaxBCG color- and density-selected galaxy clusters. He showed that the lensing, when averaged, produces a (mildly transformed) cluster–mass cross-correlation function. This can be ratioed with the cluster–light cross-correlation function to produce the average mass-to-light ratio as a function of scale, for cluster subsamples and their environments. At large radius this mass-to-light ratio asymptotes to a universal value, which leads to an omega matter of 0.2 +/- 0.2 with a systematic error (from the fact that the large-scale variance and bias are not perfectly known) of about 20 percent. Great work!