Well, it looks like people are finally realizing that being thin is not actually a healthy thing.  go figure....
but it actually goes further than that.  just maybe, the scientist have gotten it wrong too?
A recent study shows a lower death rate for people that are over weight (note: this does not mean OBESE (BMI>30.))
The reasons for this are unclear but my guess would be that a little extra energy reserves provides a survival benefit from illness.  Again, it is important to note that this was a lower death rate when compared to the "ideal" body weight.  Under weight and Obese people still have higher death rates.  Also, this study only looked at deaths, not at morbidity (or decreased quality of life that may be caused by excess weight.  It's still to premature to make a huge change in our recommendations but this may be the beginning of an upper change in the "Ideal" body weight.  Check out these links:

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

Cause-Specific Excess Deaths Associated With Underweight, Overweight, and Obesity- JAMA