Whether hydrogen or battery power, I don't understand why the roof, hood and trunk of every car can't be made out of photovoltaic material. Even if it was a secondary power supply, it's free energy. Think about how many cars are sitting in parking lots every day. My car is sitting in the parking lot at work 45-50 hours a week, during the peak hours of sunlight.
Combine this kind of solar charging with in-home charging stations, the technologies in current hybrid vehicles such as regenerative braking, EV charging stations or the type of battery change station Andrew mentions, and other technologies either already released or in the pipeline and we're not that far from the potential of a mass produced 60+ mpg vehicle. A lot of these technologies are already coming down in price or will be in the near future, others could if companies got serious about developing them for a mass market. The potential is there. The question is whether the car companies have the willpower to give us these vehicles. With gas prices going how they are and with no reason to believe the trend won't continue, the market will be there if they got serious about this.