In the posts on humanism below, I quoted and supported the idea expressed by Richard Norman that theists, atheists and agnostics (and others) should work together for common causes, even if they don't quite see eye to eye on everything. So it gives me great pleasure to point you all towards Greg Boyd on his vegetarianism:
Almost immediately after making this pledge I began to understand why the Lord had wanted me to make it. Scripture says a little yeast leavens all the dough (1 Cor 5:6). Well, I discovered that the little yeast of my willingness to engage in violence towards animals and other creatures for self-serving reasons (e.g. appetite, convenience) was polluting my heart and to some degree compromising my capacity to love. It felt like – and still feels like – my commitment to total non-violence has had, and is yet having, a purifying effect on my heart.
Obviously, my reasons for not eating meat are different to his, but there does seem to be a considerable overlap in our attitudes.