QUOTE(FlaSoxxJim @ Aug 4, 2005 -> 11:36 AM)
Hopefully the 25% number is not overly optimistic. By 2010, hopefully hybrid technology will be viewed as the stopgap green technology and we are by then actually beginning to see a hydrogen vehicle fleet.
A hydrogen vehicle fleet basically does nothing yet...
In order to make a hydrogen vehicle fleet work, you need a source for the hydrogen. Right now, the only way to get hydrogen is by burning fossil fuels. And thanks to that annoying 2nd law of thermodynamics, you always lose a lot of energy when you add in the extra step of processing the fuel into hydrogen.
Hydrogen vehicles are only practical environmentally or economically if you can come up with a renewable source of energy capable of supplying that much hydrogen. Without that, it simply chews up even more fossil fuels.
Also, one of my professors here has done a significant amount of work showing that significant amounts of hydrogen would probably be lost during transportation/storage of the gas, and hydrogen could turn itself into a pretty nasty greenhouse gas if it was used on a wide scale. (link)