Cellulosic ethanol is made from the fibers of plants. It does not require the use of cropland.
Switchgrass is the major biomass material being studied today, due to its high levels of cellulose. Cellulose, however, is contained in nearly every natural, free-growing plant, tree, and bush, in meadows, forests, and fields all over the world without agricultural effort or cost needed to make it grow.
It can also be made from wood scrap, agricultural leftovers, and more.
Earth2Tech has a highly informative post profiling eleven companies currently building cellulosic plants in the US, including Bluefire Ethanol, who plan to create ethanol from landfill waste.