Ideal body weight is not as important as optimal body fat. For most of us this is not a major issue, but weight trainers can be 50 pounds overweight by traditional body/height tables, yet actually have ideal body composition.
This new scanner works by simultaneously building up an accurate 3-D image of the body, while measuring the body's effect on an electromagnetic field. Combining the two measurements allows the researchers to work out the distribution of fat and water within.
It seems that this might be the best technology to measure body composition but the article doesn't indicate when it will be available.
New Scientist September 3, 2003