I've been a vegetarian for around six years, and vegan since June. Mainly I chose this path for moral reasons; I feel that specieism (sp?) is just as wrong as any other form of prejudice. However, I knew that wasn't the only argument one could make for veganism. One of those other reasons is almost as important as the morality argument; our planet.
A paragraph from the AlterNet post sums it up nicely:
...the University of Chicago... noted that feeding animals for meat, dairy, and egg production requires growing some ten times as much crops as we'd need if we just ate pasta primavera, faux chicken nuggets, and other plant foods. On top of that, we have transport the animals to slaughterhouses, slaughter them, refrigerate their carcasses, and distribute their flesh all across the country. Producing a calorie of meat protein means burning more than ten times as much fossil fuels -- and spewing more than ten times as much heat-trapping carbon dioxide -- as does a calorie of plant protein. The researchers found that, when it's all added up, the average American does more to reduce global warming emissions by going vegetarian than by switching to a Prius.
I think a great resource for this wealth of information is Factoryfarming.com. Please take some time to read through it.
AlterNet's "Vegetarian Is The New Prius"