About what fraction of adult Americans are overweight?

It depends on who's numbers you want to believe. Like most statistics, you can find pretty much any answer you wish, if you look hard enough. Two examples:

In January 2010, the New York Times reported "Nearly 34 percent of adults are obese, obese children tripled to 17 percent."

MedicineNet reports that " 63.1% of adults in the U.S. were either overweight or obese in 2009. That was a small but measurable increase from 62.2% the previous year." , citing a 'The Gallup-Healthways Well-Being Index'