good point Wolfgang - Germany made oil from coal in WWII and we can certainly go back to it.

CarolC - I went back and re-read the Scientific American article and
it was quite exhaustive in its study of environmental impact of each potential fuel. (There were quite a few possibilities - Internal combusion gasoline engines, IC natural gas, IC hydrogen, ..
and FUel Cell hydrogen - electrolysis from grid, Fuel Cell Hydrogen- natural gas reforming, Ethanol Fuel Cell.

Ironically one of the worst in greenhouse gas increase- was HFCell using Hydrogen from natural gas reforming. Ethanol fuel cell had the least impact.

Trouble is where do you get the hydrogen - the easiest and least expensive is natural gas reforming, but then economically its much better to use that natural gas in high effiecient powerplants that burn natural gas.

If Hydrogen is acquired from say electrolysis, this would be ideal but is quite expensive, where do you get the electricity ? IF its from the power grid - you may be better off just using an electric vehicle.
(the only issue is recharging time)

Ideally the electricity would be from a renewable source say a windfarm - but you need 3cents per kw wind - the only places that you get that is in isolated areas in central US.. Then how do you transport the electricity - either build powerlines (costly!) and some power is lost through resistance, or convert to hydrogen (electrolysis)and transport hydrogen. Pipeline isnt such a good idea
though as hydrogen is not dense and would need pipeline with a circumference of an airplane fuselage. THere is an energy cost for compressing it. Tanker trucks arent that great either - as the equivalent energy of one tanker of gasoline would require 9 tanker trucks of hydrogen. (3 for liquid hydrogen but there is a cost to compressing it)

Also the trouble with fuelcells is not a question of economies of scale- there are still a number of factors that need to be solved
ie. where do you get the hydrogen (above), storing it (even Metal hydride storage has problems) - when it burns (and it burns at a much wider range of density than say other gases or gasoline) its invisible. The Nasa safety handbook suggests holding a broom to the source to see if the straw catches fire.
Basically, there are a few hurdles and a lot of research still has to be done to make HFCells practical . Whereas on the other hand, the gas/electric hybrid is already available now. (ANd in terms of environmental impact - ranks about the same as a hfcell vehicle- which will take years and a lot of investment to make)

Not to say that I dont like the idea of a hydrogen economy, but there actually may be better uses for fuel cells, and probably the market will ultimately help decide.

Or there may be other soluticons, Craig Venter of the human genome project is working on a microbe that would eat co2 and make hydrogen.
I dont know if thats good, or scary,