I want to pose this thought, well thoughts.
Nuclear power plants have enormous cooling towers. Those enormous cooling towers release steam, right? Well steam is water as we all know. Now wouldn't water vapor also act like a green house gas, absorbing and trapping the heat.
Hydrogen cars would theoretically release water vapor which would add to water vapor amounts in the atmosphere. Imagine every car being hydrogen powered. I'd think that in some of the major metropolitan areas, they'd wind up seeing an increase in the amount of rain that falls, and the increased water vapor could also potentially trap heat. Also in the Northeast, and places that have a winter, the water vapor coming from the hydrogen cars would probably ice up the roads. Icier roads would mean an increase in salt/deicer usage. An increase in salt/deicer usage would mean pollution from it, and an increase in municipal budgets.
Now personally I'm not sure about water vapor as a greenhouse gas, and I'm all in favor of decreasing the amount of CO2 in the atmosphere, and cutting the down on the use of fossil fuels. Hydrogen powered cars could also pose unintended consequences.
Once again to clarify my stance, I'm all for nuclear energy and the hydrogen economy. I just want to see what people can come up with for possible unintended consequences.
Note: I did not stick numbers anywhere, I'm sure that the CO2 is worse than water vapor as a green house gas.