Update 2012-08-30: old graph links decayed; replaced with other graphs of same data.
Almost as many obese as healthy weight adults in the U.S., and the rest are overweight. Something changed starting about 1980.
Data source: Centers for Disease Control and Prevention (2005) Health, United States, 2005.
Graph source: Wikipedia Blue Cross Blue Shield, with this accompanying text:
While the percentage of the U.S. population considered overweight has been stable since 1960-62, the percentage considered obese has more than doubled.