Ultraefficient Urban Farming?

Stewart Brand has a very bold article appropriately titled How slums can save the planet about the benefits (while admitting some of the real problems) of highly compacted urban environments. One paragraph about urban farming really struck me and I’m very curious to hear whether it’s actually accurate and sustainable. If so it makes for some very interesting possibilities:

One idea that could be transferred from squatter cities is urban farming. An article by Gretchen Vogel in Science in 2008 enthused: “In a high-tech answer to the ‘local food’ movement, some experts want to transport the whole farm shoots, roots, and all to the city. They predict that future cities could grow most of their food inside city limits, in ultraefficient greenhouses… A farm on one city block could feed 50,000 people with vegetables, fruit, eggs, and meat. Upper floors would grow hydroponic crops; lower floors would house chickens and fish that consume plant waste.”