Probably the world's number one example of urban farming is Shanghai: "The magazine Urban Agriculture describes how Shanghai has in effect created a nutrient recycling zone around the city. The municipal government manages 300,000 hectares of farmland to recycle the city’s night soil. Half of Shanghai’s pork and poultry, 60 percent of its vegetables, and 90 percent of its milk and eggs come from the city and the immediately surrounding region. " (source: http://www.earth-policy.org/Books/Seg/PB2ch11_ss4.htm)

Of course, you're not going to this kind of scale from scattered community gardens.