You can't have fuel cells without fuel. Producing, storing and transporting those will be severe bottlenecks, particularly with hydrogen, even if the price is right.
I don't think this is the answer. If it is, don't we look silly - the answer was behind us all along.
On a more serious note, I'm keeping an eye on biotechnology for this problem. Yo, Gene Expression crew - it's time you guys whipped us up a germ that'll excrete hydrogen...
Even if they did so, there's still the question of where all the hydrogen is going to come from. Yeah, we have plenty of water, but it's not easy to get the hydrogen from that. Making it from fossil fuels is possible, but there are probably better alternative uses for those.
I'm not sure what's the next most prevalent, but off the top of my head I suspect it's animal and vegetable fats. This sounds like something a Greg Hlatky would know about. We hydrogenate fats all the time - can we dehydrogenate them and make it worth our while?