There are community garden type efforts with this in mind i think. I would love to see more people learn to be self sustaining while beautifying the cityscape. Practically speaking though, I'm not sure it would take off. It would require millions of dollars in restructoring roads and sidewalks to allow for proper soil and water irrigation, in those areas. Growing food is very labor intensive take it from someone who has tried and failed even here in the country. I don't have a green thumb to save my life. Are you thinking instead of the decorative trees and bushes you sometimes see around town there be apple trees or what not? I think for the most part those plants are chosen because they are very low maintenance, are breeded to thrive in various weather conditions and can withstand the air pollution and foot traffic that they endure daily. Fruit trees require lots of pruning, fertilizing, harvesting, watering, pretection from pests etc. And even if you got some fruit from it it would be picked bare in a matter of hours or days for the months of work you'd put into it. I think to solve the food shortages our country needs to first stop wasting food. Tons and tons of perfectly good food ready to eat is dumped out daily. I like your thinking though! Keep the ideas coming!