I'm a big biodiesel fan, and bought my diesel car with the intention of running biodiesel in it as soon as the warranty runs out. (And hopefully by then, there will be enough evidence that biodiesel in a common rail diesel engine isn't a big problem)
Biodiesel is a very good option, but don't generate it out of soybeans. There is a better way,
biodiesel from algae. (Or if you prefer, here is the
government report (PDF) in all it's glory)
While it would still take a lot of land to produce and equivalent amount of energy to all gas+diesel combined (15,000 square miles), it is nothing compared to the amount of land used to feed livestock.