My personal opinion is that there will come a day when there will be lots of metal structures lying around (cars, boats, planes, etc) as there will be no fuel to propel them.
True, alternative sources like gasohol are being experimented with, but most of these and other renewable sources are insufficient or not practical to meet the demands of the exponential global population growth.
For example, gasohol is approximately a 9:1 ratio of gasoline:alcohol mix. But alcohol is obtained from fermentation of crops like corn. Corn needs to be grown and harvested. This is apart from the moral question of sacrificing food(corn) for fuel; food which could have been used to feed third-world populations instead of being fuel for aircraft.
Perhaps the best solution to a fuel crises will be nuclear power. Would anyone feel safe in a nuclear-powered cessna?
But I believe we who read this thread today will be long dead before such a day comes.