Sciencegeek, I'm pretty sold on the arguments that I've read for the notion that plant based diets are better for the environment. Based on certain studies the UN have been advocating for people to adopt veganism for a few years now:

http://www.youtube.com/watch?v=tQlekfaPyaA

http://www.theguardian.com/environment/2010/jun/02/un-report-meat-free-diet

Quote: "As the global population surges towards a predicted 9.1 billion people by 2050, western tastes for diets rich in meat and dairy products are unsustainable, says the report from United Nations Environment Programme's (UNEP) international panel of sustainable resource management.
    It says: "Impacts from agriculture are expected to increase substantially due to population growth increasing consumption of animal products. Unlike fossil fuels, it is difficult to look for alternatives: people have to eat. A substantial reduction of impacts would only be possible with a substantial worldwide diet change, away from animal products."
    Professor Edgar Hertwich, the lead author of the report, said: "Animal products cause more damage than [producing] construction minerals such as sand or cement, plastics or metals. Biomass and crops for animals are as damaging as [burning] fossil fuels."

That said, I'm not much of a science geek myself. So I don't know how great the science is that the UN's report was drawn from. CS