Just ran across this article:
http://blog.wired.com/cars/2008/06/bmw-builds-a-ca.html (hope the link works). I've been puzzled at the lack of interest here in ways to lighten a car, and this should do it. Would save on minor body repairs, too: instead of expensive body shop work, you just zip in a new panel :-)
It's not an entirely new idea, either: up until WWII, most airplanes were built with fabric over tubing (or even wood) frames.