Linköping, Sweden, a city of about 104,000 people is considering a vertical farm project to become self-sufficient in food (http://www.good.is/post/a-vertical-greenhouse-could-make-a-swedish-city-self-sufficient/) while Chicago is already building their first vertical farm (http://www.plantchicago.com/) and Berlin is planning the world's largest rooftop fish and vegetable garden (http://www.treehugger.com/sustainable-agriculture/worlds-largest-rooftop-fish-and-vegetable-farm-planned-berlin.html).
In the US, Growing Power (http://www.growingpower.org/) in Milwaukee is probably the most successful urban gardening project. Today, 1% of the food consumed in Milwaukee is grown in the city but Growing Power wants to increase it to 10% within two years. They plan to build 100 acres of greenhouses for year-round growing and have begun a 20,000 backyard garden program. You can see their founder, Will Allen, talk about their work on CSPAN's Book TV (http://www.booktv.org/Program/13443/The+Good+Food+Revolution+Growing+Healthy+Food+People+and+Communities.aspx)