I've begun to catch up on reading articles and reports, and want to recommend this one from Brighter Planet
http://attachments.brighterplanet.com/press_items/local_copies/52/original/carbon_foodprint_wp.pdf?1264533289
The American Carbon Foodprint is a well-written and nuanced report. There's not much that's new here, but there are lots of good graphs illustrating the various points. There's also a list of seven directions to take in reducing one's contribution to greenhouse gases from food. (That sounds strange, but you know what I mean.)
The one question I have of this report is that it states that conventional agriculture is more productive, sometimes much more productive, than organic agriculture, which can mean that per calorie, conventionally grown foods contribute less in CO2 equivalents to the atmosphere. I wonder about this. Are they referring to large-scale organic or unsustainable organic? Because some studies have revealed that small-scale, diversified, sustainable organic farming has equivalent or higher yields to farming based on synthetic fertilizers, pesticides, etc.
If you are interested, it's not a long read. If you only have a moment, go to page 17 for the list of seven things to do.