Ain't gonna happen.
Here's a fascinating article by engineer Robert Zubrin from the journal The New Atlantis that explains in detail why the vision of cheap, efficient, clean hydrogen-driven cars humming down our highways is a pipe dream at best, a hoax and swindle at worst. And, as Zubrin says, "Incredibly, the Bush administration swallowed it, hook, line, and sinker."
The crux of the matter is that while hydrogen can be used as a fuel, it is not an energy source like petroleum. Burning petroleum returns so much more energy than it takes to extract that it's basically free energy. Technically I suppose oil could be seen as millions of years of sunlight from eons past stored in the earth for us to find, and in chemistry the energy scales always balance. But on any human scale it's just free energy waiting for us to find it.
But there's no natural source of pure hydrogen on earth. It has to made, and any known process consumes much more energy than you get back when you use it as fuel. So it's hopelessly wasteful compared to oil in energy terms, and then there's the practical problem of distribution. Hydrogen is dangerous stuff, as some zeppelin passengers found out in 1937. Accordingly, the economics of any conceivable hydrogen fuel system quickly becomes ludicrous. Would you want to pay $1,000,000 for an inefficient car that was likely to go up like the Hindenburg at the first fender-bender?
We do desperately need to find a solution to our energy problem. But hydrogen isn't it.