FWIW, I'm a little unsure about the value of buying local for a couple of reasons.
I'm not particularly advocating buying local as a better option, it was just an example of the kinds of factors that one might need to consider.
I tend to think that if there is a significant negative externality to carbon emissions that is not currently reflected in prices, the optimal solution would be to impose some kind of carbon tax to reflect that hidden cost. This would avoid the need for individuals to try and make complex cost benefit calculations for themselves on optimal carbon reducing choices.
I don't think it's very likely that it is politically feasible to implement such a tax though so if I considered the issue important I might attempt to make lifestyle choices that reduced my own personal impact. Under those circumstances I'd want to make choices efficiently. It's not clear to me that vegetarianism would be the best choice but since I don't consider reducing my own carbon footprint a priority I haven't done a lot of research on the issue.
It's not clear to me that vegetarianism would be the best choice
Given that vegetarianism doesn't exclude other strategies for emissions reduction, I'm unclear why you think it's relevant whether or not it's the single best strategy. Surely all that's required is that it have a net positive effect?
I have noticed that among philosophers, vegetarianism of one form or another is quite common. In fact, I became a vegetarian (technically a pescetarian) myself partly out of respect for an undergraduate philosophy professor. I am interested in finding out if there is a similar disproportion in the Less Wrong community.
I didn't request that this go into Yvain's survey because I want more information than just what animal products you do or don't eat; I'd also like to see nuances of the reasons behind your diet. There are a lot more shades than carnivore/vegetarian/vegan - if you want to be a vegetarian but are allergic to soy and gluten, that's a compelling reason to diversify protein sources, for instance. I'd also like to hear about if you avoid any plant foods (if you think they're farmed in a way that's environmentally destructive or that hurts people or if you have warm fuzzy feelings for plants, maybe). Here are some questions that come to mind: