Steve and others.
While I completely agree that giving up meat is a tremendous thing everyone should do for their health and the planet, where the environmental benefits of eating really become interesting is when we switch away from grains to a fruit-based diet.
If we replaced our fields of grains with orchards, not only would we be not adding large amounts of C02 to the atmosphere via animals, but we'd be absorbing it, we'd start adding to the topsoil layer instead of reducing it, and our major water problems would be considerably reduced.
Check out this article:
Save The Environment: How What You Eat Changes The World.