Don't hold your breath waiting for one either. You can run either straight gasoline or pure hydrogen as fuel, but not a mix. To get the most out of 87 Octane gasoline, one needs to set ignition timing around 12-14 degrees BTDC. Hydrogen has NO Octane rating and timing must be retarded to TDC. Even then there will be some knocking under heavy load conditions.
As far as being able to generate enough Hydrogen to supplement (and reduce gasoline comsumption) existing fuels I'm reasonably certain it won't work. The amount of energy required to split water into its component parts would negate the energy gained.
BMW has a dual fuel car they call the Hydrogen 7. It does all of the switching of ignition timing etc automatically. Beautiful car. It has a LIQUID Hydrogen tank that IMO would be ideal in a Fuel Cell powered car. That would probably extend the cruising range several fold.
I hope to live long enough to drive such a car.