Here's a pretty good clip of a guy that engineered his own use for hydrogen. it seems very promising.
He set up his '94 van to run on hydrogen and it went over 100 miles on a few ounces of water. I love the idea because it doesnt seem like it's Star Trek stuff here. It doesn't seem "too futuristic" to work on a large scale.
I read this in popular mechanics mag. Its the most logical solution to our gas problem. But its still years and years off. Cars wont be as powerful as they are today though which sucks.
The problem with this is often the electricity required to run the electrolisis. Now days we have power shortages in the summber because people use air conditioning. Imagine the power requirements to fuel the US's cars, and everything else.
So perhaps you've stop polution from the car, but you've created more by the need to build more power plants. The only advantage is that we wouldnt have to import water, thus reducing the up and running cost. However start up cost would be astronomical.