For those interested in the implications of losing weight, vehicle-wise, there's a great read in the new June '12 issue of Automobile Magazine outlining the measures taken with the Benz 550SL. Torsional Rigidity up 20%, 165 added lb. of safety engineering, 275 lb. NET LOSS in Curb Weight while growing in size and adding a twin-turbo V8...
Answer? Only 8% of the structure is STEEL, and that is largely HSS in the windshield frame and its support system.
Weight can disappear, but it won't leave with an inexpensive result.