it takes more energy to create the hydrogen than you get out of it. but from what i've read, im no expert on the matter, subsidizing gasoline or diesel with hydrogen could improve the combustion of the fuel causing a more complete burn. im not sure if there is a true benefit or increase in mileage but either way, you cannot just plug something into the car and expect a miracle.
not to mention the emission from combusting hydrogen is water, aluminum heads, cast iron manifold, steel exhaust... corrosion and rust anyone? how much would you save in gas? how much does a new exhaust cost?