Apart from the issues around cruelty to animals, which most people seem immune to, it seems being a meat eater is worse for the environment than driving an SUV. Methane, a product of animal farming is a much more powerful greenhouse gas than CO2. See the following links for details:
http://geosci.uchicago.edu/%7Egidon/papers/nutri/nutri3.pdf
http://www.fao.org/newsroom/en/news/2006/1000448/index.html
http://earthsave.org/globalwarming.htm