ERS has an interactive map here. (Warning, I had a problem with Firefox 3.5.3, but not with Chrome or Firefox 3.6. And you may have to "reset the map") It displays state level, and some county-level data, on various parameters relating to food: availability of supermarkets, availability of fast food, demographics, consumption per capita of various kinds of food, etc. etc.
I think I owe a hat tip to Obamafoodorama.