Early this year, when it became clear that the Chevrolet Volt and Nissan Leaf had missed their 2011 sales targets, critics declared the electric-car revolution over. Yet at Detroit's annual North American International Auto Show in January, plug-in cars abounded. BMW displayed its forthcoming i3 electric city car, along with its i8 plug-in hybrid sports car. Acura unwrapped a hybrid concept version of the NSX supercar. Tesla Motors brought its all-electric Model S sedan. But the most important car on the show floor might have been one that, on the surface, seemed much less exciting: the new Ford Fusion, which will be available in gasoline, hybrid and plug-in hybrid versions.