http://www.treehugger.com/files/2007/02/mits_stackable.php#comments

The interesting part about this MIT design, at least to me, is the potential to mass produce electric plugin conversions for front wheel drive internal combustion cars.

A kit could include two of these wheel/motor combinations that bolt on in place of the regular rear wheels and a charger/controller/battery pack that goes in the trunk.  Quick, economical  conversion of your used economy car to plugin hybrid.  The first 25 miles of your drive on battery power, before you need your gasoline engine.

That's a way to convert the world's cars to plugin hybrid without the huge expense.  A 5000 dollar kit would pay it's way in only a few years of gasoline savings.