I don't really think they will be self-sustainable, even on a small scale, but I think they can (and will) do much more than they are doing now with high-rise factory farming. Detroit, for example, has some empty places to start with. Also, these sorts of things can be done right outside of a city using smaller amounts of land - until we have more ecological transportation, that's a good start.

Once we are able to grow the protein (cuts of beef, chicken, etc) in labs, it wil get much closer. Hopefully that is better for the environment than live animals.

But then again, I do not believe people actually want self-sustainability with foodstuffs, nor do I think we should be growing everything in a self-contained community. A lot of folks in Western countries eat a wide variety of foods and spices that aren't best grown in their climates and I think it would take much more energy to grow them where they simply don't thrive. If we get far enough with GMO's, this could be solved.