The Wall Street Journal had a piece in their Personal Journal section about Tony Gonzalez, KC Chiefs football player, and his quest to maintain a vegan diet. (The only link I can find online is here go to the right side of the page to see him make a vegan shake)
The thing that I have to question about a vegan diet is this. If you are to eat as a vegan, you need protein from a source other than animal products. Apparently, you cannot consume enough protein from beans and nuts to satisfy a human's protein dietary requirements. So many vegans have to eat some kind of soy based product like tofu or milk. Now mind you, it's not like you can run over to the local health food store and buy a couple of pounds of soybeans to eat as a snack (maybe you can). Soy products all have to be processed into something else.
Obviously, cavemen didn't have the ability to sit down with a veggie burger and a big ole glass of rice milk. If they had to live on berries, nuts and beans indigenous to the area they would have starved to death. So I have to wonder about the propriety of such a lifestyle.
All that aside, I always go with "if God didn't want us to eat animals, he wouldn't have made cows taste so good".