Examples of urban farming, in the form of backyard gardens have existed in New York and large cities for a long time. New examples exist of this turning in to a profitable activity with low income families selling food to there neighbors. This fresh produce has health benefits for people that often don't get fresh food.

Normally the soil in a city environment that has seen construction is viewed as unusable agriculturally, but some research has shown that if the pH level is kept neutral it can be cultivated.
http://www.nytimes.com/2008/05/07/dining/07urban.html?pagewanted=3&_r=1&ei=5070&en=9d6a23b0418d45a4&ex=1210824000&emc=eta1

See New York Times slide show on gardening in the city
http://www.nytimes.com/slideshow/2008/05/07/dining/0507-URBAN_index.html