In the past three days, I've seen or heard of hydrogen cars at least three times. It's a promising technology and its long-term implications are tremendous. The BMW model is pictured to the right; you can see a video from the BBC describing GM's prototype -- not only is it hydrogen-powered but has some other interesting features.
As I've read elsewhere, it takes a lot of energy to create energy, and this would be no different than the energy it takes to pump oil out of the ground, move it to a refinery, turn it into gasoline, and then distribute it to tens of thousands of local stations that sell it. Extracting hydrogen from seawater will be the same. There is also the problem that hydrogen can't be compressed, and there is as of yet very little infrastructure to distribute it.
But, hydrogen promises to be somewhat cheaper than gasoline, the environmental impact of widespread adoption of hydrogen engines would be tremendous, and most of all, it would significantly diminish the geopolitical significance of oil-rich regions of the world. Just like my previous hopes about the nearness of efficient oil-shale extraction technology, there is the danger that this is putting all my hopes in a gee-whiz technology basket instead of urging the adoption of more funadmental kinds of reforms and changes, but at least now there is some diversity of such baskets.
I still think it bears some thinking about what the world would look like in fifteen years or so if the energy axis of the world is tipped so far to one side. Petroleum power would not go away, either in the industrialized or the developing world. But with the U.S. as a net exporter of petroleum derived from Colorado's shale, OPEC broken, and the primary source of motive power for individual vehicles shifting from gasoline to hydrogen, the U.S. would regain a substantial amount of hegemony which we currently feel slipping away. At the same time, energy policy would diminish in importance, and geopolitical alliances currently formed because of petroleum rivalries would dissolve. Countries would start finding other reasons to jockey for position and alignment. Religion would be one such axis; so would access to the ocean (as a source of seawater for hydrogen-powered vehicles). Refinement and extraction plants would also become very important. Landlocked nations would become dependencies on their neighbors or would begin to deplete their own freshwater resources. Environmentally, it's likely that the world would become somewhat more humid with more water vapor and heat emitted into the atmosphere, and continued greenhouse effect from use of petroleum-driven engines and the massive amounts of coal used to power plants that power the electrolicization of hydrogen plants. Environmental changes would not stop; they would slow perhaps, but we would also see more examples of extreme weather like super-hurricanes.
Perhaps this world would best be explored, for the time being, through the vehicle of near-future science fiction. Ah, another writing project I've invented for myself.