The Boeing demonstrator uses a Proton Exchange Membrane (PEM) fuel cell/lithium-ion battery hybrid system to power an electric motor, which is coupled to a conventional propeller. The fuel cell provides all power for the cruise phase of flight. During takeoff and climb, the flight segment that requires the most power, the system draws on lightweight lithium-ion batteries.
Clearly Boeing never thought that they are even close to powering up planes with fuel cell technology, a small singly manned air plane is all that is being tested currently. However Boeing do hope that in 10-15 years time, secondary systems can be powered by such hybrid means saving on fuel. Humanity does have a long way to go before generating power equivalent to the jet engines using environmentally friendly means.
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Powering a car with a fuel cell which can continously charge the battery and give us enough range as well as top speed still seems to be some way away, but that hasnt stop the engineers from Boeing who believe that air planes sans fuel must be invented asap. I agree with them on the logic, however I do wonder if 2007 is a bit too early to create planes which could fly on fuel cells. The fuel cell generates electricity to charge the batteries utilising hydrogen and not producing any heat in the process (since there is no combustion).