Maybe the air flowing through the grill and under the car could be harnessed via fan blades that then recharge the battery. This could be self defeating if the drag created or the additional weight out weighs the beneft. Is it worth a look?
I think there should be a sticky on this forum somewhere entitled: "Why you can't just stick a wind turbine on the hood of your car and drive for free." It's cool that lots of people out there are putting their thinking caps on and looking for non-conventional solutions, but since most people aren't engineers and physicists, they need a little guidance. This thread topic is getting a little tired for some of us old timers, and we still have a year and a half until we can get our Volts!