From the New York Times: Causes of Death Are Linked to a Person’s Weight - New York Times

Linking, for the first time, causes of death to specific weights, they report that overweight people have a lower death rate because they are much less likely to die from a grab bag of diseases that includes Alzheimer?s and Parkinson?s, infections and lung disease. And that lower risk is not counteracted by increased risks of dying from any other disease, including cancer, diabetes or heart disease.

As a consequence, the group from the Centers for Disease Control and Prevention and the National Cancer Institute reports, there were more than 100,000 fewer deaths among the overweight in 2004, the most recent year for which data were available, than would have expected if those people had been of normal weight.

Their paper is published today in the Journal of the American Medical Association.
This kind of fits in with the earlier thread about drinking water. As researchers look more closely at commonly held health beliefs, they find more and more reason to doubt the conventional wisdom.

BTW, here's a quick little body mass index calculator.