Vertical farms may be pie in the sky, but they are a lot of fun for architects with time on their hands. AWR recently organized a competition to design a vertical farm with a residential use called LOFT 2011.
I am particularly fond of the Second Honorable Mention, the Breathing Vertical Farm. There isn't a lot of growing area, but it would make a beautiful place to live. See the whole entry here.
The winner was VAWA, or "Vertical agriculture with architecture." The jurors do not issue a statement of why it won. See the full entry here
Second Prize went to Vertical Fields. It is built from four towers with a small enough footprint it could probably be lit naturally.
It would be quite dramatic looking as well, sitting beside Norman Foster's London City Hall. See it here.
Third prize went to the Cultivated Carousel, where somehow trays of plants rotate on giant wheels. See it here.
Like most vertical farms, these proposals are long on visuals and short on reality; a working vertical farm would be a lot more utilitarian, like Gordon Graff's master's thesis. But there's nothing wrong with a bit of lovely eye candy now and then.