In class, I talk about how hard it is to figure out which activities are good for the environment, once the entire supply chain is factored in. And how common strategies to reduce your footprint are often dumb. In class, I talk about how ridiculous policies like banning bottled water (ala San Francisco) or banning air conditioning (Japanese government) are just incredibly dumb.
Part of the problem is that once you calculate the entire supply chain, it is unclear what practice is better for the environment. This recent nytimes article argues that steel water bottles are better than disposable plastic if you use it at least 50 times, though if you read the article carefully, if you wash your water bottle with hot water, then there is no benefit at all. The same is true for local food. Though you are not shipping your produce from Mexico, the fact that local food is often brought in smaller trucks and you drive more to buy it, means it is often worse for the environment.
The other part of the problem is that there are much cheaper and simpler ways to help the environment. For example, for about $20 you can take a ton of CO2 out of the atmosphere. Companies like Terra Pass will take your money and use it to plant trees or reduce landfill emissions or buy more fuel efficient stoves in poor countries.
So yes, you can reduce your carbon footprint by about a ton a year if you give up meat, or you can achieve the same effect for $20. If you value meat at less than $20 per year, than go ahead, but otherwise, that's not the best approach. There are of course other reasons to be vegetarian, just like I love local food.
Given the complexities, how do we make figure out what's right. Of course the easy economic approach is to make sure you pay for the carbon you are responsible for. This can be accomplished most simply with a carbon tax, or a little less simply with the cap and trade proposals currently on the table. That way all you have to worry about is the price, which encapsulates all the environmental costs within it.