This isn't specific to urban agriculture, but I found a neat site on the topic of rebuilding small farm production:
http://serveyourcountryfood.net/
Of particular interest is the maps section. Some of them are a little old, but it provides a starting point for talking about the resources, history, and infrastructure involved in food production in the US.
BTW, there are some interesting urban-specific sites out there too, if you haven't seen them yet.
http://www.cityfarmer.org/ isn't very well organized, but full of info if you poke around. Same goes for
http://www.pathtofreedom.com/. Anyone else have related links to share?