The amount of available plant waste is so small, that there's no scenario where even 1% of current energy needs can be met by using it. We'd need several planets worth of biomass to do that.
The public would be well served to remember that our civilization did run on biofuels some centuries ago. In those days it was called burning wood, riding horses, and plowing with oxen. We have since abandoned the model, precisely because it was severely limited in available energy output.
We can improve on that by research, but there are fundamental thermodynamic limits on the energy conversion efficiency of biological systems. Those limits are far below other methods of harnessing solar energy. A good example is the comparison between biofuels (e.g. from the best crop we have - sugarcane) and PV-electrolysis, where the former has a ~0.5% efficiency of solar to fuel conversion, and the latter about 10% (solar to hydrogen). It would be completely self-deceiving to think that anything we do with algae or any other biological system can improve the conversion efficiency by 20000% - and that would be just to catch up with electrolysis. Other methods have the potential to convert sunshine to hydrogen at >30%, roughly an order of magnitude more than what one can even theoretically achieve via photosynthesis.

@physics314 - Unless there have been some dramatic and unpublished updates to electrolysis it is a red herring. It either uses expensive platinum group metals which foul and require reprocessing, or more common metals like aluminum and copper which are consumed in the reaction. So to put it briefly, electrolysis literally burns metal to power the hydrogen production you'd be better off skipping hydrogen and just using batteries that consume aluminum plates and swap the plates when they are fully oxidized.
Thermally breaking water is a better solution to hydrogen production. At a few thousand degrees (~2500C) the hydrogen/oxygen bonds in water break. This is easily achieved with a nuclear heat source. Thorium reactors could meet the portable hydrogen demand easily and thorium is sufficiently plentiful to meet that need for the next several hundred thousand years. As an added benefit radioactive waste can produce hydrogen from water directly as well. It is impossible to render hydrogen radioactive so the hydrogen would be clean and usable. This would mean that the minimal waste produced by the thorium reactors (Something like 1/100th that of a traditional reactors)
It looks like there is an electrolysis method that uses dry steam and electricity applied without collection plates at high temperatures that can electrolyze water, but it generally requires a nuclear reactor to heat the water to steam and power the electricity for the reaction so why not just thermally separate the water? (http://www.acs.org/content/acs/en/pressroom/newsreleases/2012/march/nuclear-power-plants-can-produce-hydrogen-to-fuel-the-hydrogen-economy.html)
For those who are so scared of the word "nuclear" they can't be bothered to research thorium and understand its safety and lack of waste products there have been efforts to use concentrated solar to perform thermal separation, but I think they had issues with scaling and oxidization as well as the issue of requiring insanely huge swaths of permanently sunny land. So you are trading a minimal nuclear waste stream for permanent destruction of delicate habitats like deserts.

But as G.W. said, the American way of life is not negotiable.

Nuclear and Hydrogen are both highly explosive. Nuclear even sounds scary when George Bush says it! Please, stick to nice safe gasoline; and get rid of those dangerous pipelines. Transport highly flammable fuels by tanker cars, they way god intended.

"turns corn stover into hydrogen and carbon dioxide"
Isn't CO2 something we want less of?

"turns corn stover into hydrogen and carbon dioxide"
Isn't CO2 something we want less of?
No,you are just returning CO2 recently sequestered by the plant waste to the atmosphere,which is why the process is called carbon neutral.Burning fossil fuel, in contrast, releases millions of tons worth of stored CO2 in a matter of decades.

As physics314 said, there isn’t enough biomass available to produce a significant percentage of the world’s energy needs via this method of hydrogen production. So, that isn’t the point of this article.
But one company could make a profitable venture based upon this technology, if the claimed efficiency is achieved. Notably, vast amounts of plant waste is now hauled away as garbage. Usually, this biomass garbage is a nuisance. The owner of the garbage must spend time or money to get rid of it. So a new company could obtain the biomass for free (or be paid to haul it away.)
Similar methods have been attempted by Verenium Inc (now a division of BP). They have constructed a plant adjacent to a ethanol production plant that converts sugarcane sugar into ethanol. The sugarcane factuality had been hauling away the bagasse (cellulosic parts of the sugarcane plant). Now Fuel Cell Inc using its cellulose-digesting enzymes to convert the bagasse into ethanol. This method is more cost efficient only because the costs of acquiring the raw material have been eliminated and shipping costs for moving the raw material. And it keeps this biomass out of landfills (which are rapidly filling up.)
The cost effectiveness derives from minimizes costs throughout the entire process of acquiring the biomass to production of fuel (hydrogen, methanol, or ethanol.) Similar plants have been constructed near timber mills to render sawdust and wood chips into ethanol. More at: http://www.biofuelsdigest.com/bdigest/2012/08/17/why-sugarcane-bagasse-is-the-most-promising-pathway-for-cellulosic-ethanol/
Or search for 'verenium ethanol cellulosic ethanol'

If water can be "thermally" broken into hydrogen and oxygen, would it be possible to focus solar energy into a flow of water and extract the component molecules directly? That is, without starting a hydrogen fire?
Wonder if charged anode and cathode might polarize the molecules in different flow directions, and prevent recombination? Just thinking out loud.

Some of these criticisms are merited but still miss some important points. All cities produce large amounts of municipal trash and sewage which have to be disposed of with results that are mostly, well, crap. So, work out a solution that consumes sewage and at least the biological content of municipal trash. The first important objective is to effectively and cleanly process waste. The secondary objective is to produce as much hydrogen as we can as well as any other useful material such as recovering phosphorus. If this can be done at a modest stable capital cost the recovered energy or materials help soften the cost of disposing of waste.

There are a number of reasons why hydrogen is anything but a viable fuel for transport, ranging from the necessity for cryogenic storage through poor energy density to effects on the confining structure such as hydrogen embrittlement, to say nothing of its explosive potential.
Now, if instead of the enzyme producing hydrogen and carbon dioxide - a distinct drawback in its own right - it could be persuaded to produce hydrogen and carbon MONoxide - AKA syngas - which could be used as feedstock for the good old Fischer-Tropsch process, then we would be cooking with gas!