Interesting posting at the NYT's Green Inc. blog. First cites 2006 UN report that livestock is responsible for 18 % of greenhouse gas emissions. But then a recent Archives of Internal Medicine editorial stating that water use is 2-5 times greater worldwide for animal-source food than basic crops such as legumes and grains. Also states that livestock production accounts for 55% of the erosion process in the U.S. and 1/3 of the total discharge of nitrogen and phosphorous to surface water.
See the blog and links at http://greeninc.blogs.nytimes.com/