Flickr member Holgalicious had fresh tomatoes on her Chicago rooftop again this past summer. What she has done can be easily replicated on city rooftops all across the country using sub-irrigated planters (SIPs, aka "self-watering") like these EarthBoxes.
All that is needed is better education about the benefits of urban gardening in SIPs of all kinds. Ask your local botanic garden and extension program educators why they are not offering workshops about SIPs. Their reply would be of interest to all of us.