Last week's issue of New York had this exciting piece about "Skyfarming":
Why build vertical farms in cities? Growing crops in a controlled environment has benefits: no animals to transfer disease through untreated waste; no massive crop failures as a result of weather-related disasters; less likelihood of genetically modified “rogue” strains entering the “natural” plant world. All food could be grown organically, without herbicides, pesticides, or fertilizers, eliminating agricultural runoff. And 80 percent of the world’s population will be living in urban areas by 2050. Cities already have the density and infrastructure needed to support vertical farms, and super-green skyscrapers could supply not just food but energy, creating a truly self-sustaining environment.
The article outlines how it would work--everything from how the skyfarm would be powered (solar and wind), organized (cylindrical to maximize light), and sustained (an "evapotranspiration recovery system" of pipes would collect plant water vapors and a filtration system would clean that water, yielding gray water, which while not drinkable, is good for irrigation). I don't know enough to evaluate the whole project, but it seemed pretty convincing to me.