I suggest that anyone who is interested in this topic check out the book The Vegetarian Myth by Lierre Keith. She essentially deconstructs the arguments in support of vegetarianism from three realms: biology, politics and ethics.
In regards to ethics, the arguments in support of vegetarianism can be dismantled from numerous angles. While vegetarians are justified in their disgust and horror over factory farming, the leap to disregard all meat eating as unethical often goes unexamined. The existence of vegetarianism can in many ways be traced to our increasing alienation from the world of nature. Keith suggests that vegetarianism is often intertwined with a denial of the reality of death (not to suggest that the average SAD meat-eater is not also in denial), a refusal to accept the inevitability that our survival is dependent upon the death of others (be they animal, vegetable, fungi). Vegetarians are just as presumptuous about the lack of plant sentience as most meat-eaters are about non-human animal sentience (see: The Lost Language of Plants by Stephen Harrod Buhner, which presents a fascinating explication of the biological mechanisms suggestive of plant sentience). There is no escape from some kind of killing for our survival (good luck trying to be a fruitarian/scavenger), and only a people who get there food from the grocery store could think otherwise.
Fundamentally, vegetarianism is dependent upon the unsustainable practices of agriculture. Modern veganism, in particular, is also dependent upon the globalized industrial food chain. Vegetarianism (via agriculture) literally feeds on entire ecosystems in the service of its ideals. Animals raised on pasture (provided they are not allowed to overgraze) can actually build topsoil and enhance biodiversity, whereas agriculture erodes topsoil and consists first of all in the removal of all life from the land to be cultivated.
But to accept that agriculture is unsustainable would mean that we would also have to accept that civilization is not sustainable, that the Paleolithic (by which I mean "indigenous") is not something from which we can pick and choose. It is the only sustainable and therefore the only truly ethical social/ecological arrangement that is available to humans.