Hydrogen my dear boy,
The by product of hydrogen as a fuel is water and oxygen. Fuel cells are already being developed for cars/buses etc and they work using reverse electrolysis.
The only caveat is - the method of producing the hydrogen should be via sustainable means i.e. hydro (dams) etc. If the world starts producing hydrogen by normal means, we won't see a carbon dioxide emissions benefit.
Being as aviation is one of the greatest polluters, there are already some advances in the designing of hydrogen fuelled air vehicles. One of the main drawbacks so far is the safe storage of the fuel. If we safely stored the hydrogen fuel for an aircraft, it would, at the time of writing anyway, be far too heavy to be viable.
As you say, there are about 30 to 50 years of fossil fuels available however, as it becomes more scarse and therefore more expensive, it then becomes much more economically viable to scavenge the hard to get resources that we are currently leaving in the ground.
Therefore, there is probably going to be plenty of fossil fuels for the medium term, I just hope that we have the foresight to actively seek the hydrogen alternative quickly. It would take decades to realise the hydrogen dream, even if we ploughed limitless resources into the quest.
The benfit of benefits is - we would no longer need to poke our big noses into the middle east. We would be self sustaining in our enegy production, what a different view we would have of the world then?
Ginge.