actually, hydrogen is a pipe dream. It would be nice, but as a replacement for transportation fuels it really isn't realistic. The entire fueling infrastructure of the country would have to be rebuilt for high pressure gas, which is extremely hazardous, and expensive. Pretty much the only positive of hydrogen gas powered cars is that if the tank ruptures, the gas evaporates immediatly into the athmosphere rather than pooling like a liquid fuel. However if a tank ruptures, the explosion and intense cold could do alot of damage in and of themselves. Also, even if we started producing hydrogen as a fuel source, you have to use energy to make it, and something like 95% of the energy used to create hydrogen, comes from coal or petrol fueled plants. I think norway is using renewable sources (IE solar, wind, etc. ) to produce their hydrogen, for the highway from Oslo to.... crap i forget the other city. but America just isnt set up that way.
Something to look into, that im very interested in (im majoring in green technology) is algae diesel. It has tremendous potential, even though its in the early stages and there are obviously problems to sort out. And on top of that, the glycerine produced as a byproduct, could be passed through an APR device (aqueous phase reformer) or a plasma arc gasifier to produce a synthesis gas that can be used in the same manner as natural gas. Two fuels from one source. and totally green. no pun intended.
the future of transportation fuels, IMHO, is going to be hybrid diesel electric vehicles burning some sort of biodiesel renwable fuel. With the electric motor doing the driving, and the diesel engine only running to spin a generator to recharge the batteries from time to time on the go.