Obesity is the second-leading cause of preventable deaths in this country, according to a recent report in the New England Journal of Medicine. The government now says that 34 percent of Americans are clinically obese - at least a third of their weight is fat. Being overweight is a nationwide problem and a national obsession. Every year millions of Americans go on diets. Some of them lose weight permanently; most, however, do not. 48 Hours takes a probing look at this issue, focusing on several people trying to slim down, and on some new dieting strategies - including a controversial surgery that literally shrinks the stomach.