Hydrogen is fairly costly to produce. It comes mainly from the cracking of CH4, methane (nautural gas), coal, or electrolosis of water, which requires copious amounts of energy to produce. However, we don't need fuel cell vehicles to begin the switch over to H2 fuel. Currently, there are affordable ICE's that run on H2 with the only emission being water. Combine that with hybrid or plug-in technology. Now the problem is how do we create the refueling infrastructure to accommodate hydrogen?
One way would be to make the hydrogen right at the pump using water and some form of cheap renewable electric power, windmill, nuclear, solar, clean coal, etc... The cost per mile would probably be comparable to gasoline but dependency on foreign oil would be reduced.

Ralph