Sugar cane's efficiency in producing ethanol is 800 percent compared with 130 percent for corn, as others have mentioned (HCN, 2/4/08). Currently, our sugar cane lands in Hawaii are fallow or growing eucalyptus trees. But even if we replanted cane to all these lands and also to suitable lands in our sunny Southern states (now growing soybean crops, some of which are used to make biodiesel), we wouldn't make much of a dent in our fuel needs. Switchgrass efficiency ranges from about 200 percent to 1,000 percent depending on the process. Switchgrass outshines most other sources for being environmentally friendly (good for wildlife, very little erosion and stream sedimentation - unlike corn - requires no fertilization, and produces less greenhouse gas). But, as mentioned by others, there is the problem of land availability.
Richard Conniff, writing for Smithsonian, estimates that to produce biofuels from agricultural crops, whatever they are, would take more than two times the total area of arable land currently in existence. And with global warming, the area of arable land is decreasing. Ethanol is also being made from food-processing waste, algae, wood debris, and other forms of biomass, which may be a good thing in terms of waste management, and might reduce our total fuels needs by a percent or two. But the ultimate solution has to be a combination of various fuel-manufacturing processes, various energy-capturing sources (wind, water, solar, gravity-film heat exchange, chemical reaction), improved combustion and energy-utilization efficiency - and, dare I say it, reduced fuel consumption.