Recently, new research came out about the causes of mortality at different BMIs and weight. According to Katherine Flegal, the lead researcher of this study, if you're underweight, there is a significant increase in non-cancer and non-cardiovascular related mortality. If you're obese, there is a significant increase in cardiovascular mortality. However, if you're overweight, then you're okay because there could be some extra nutritional reserves. Hmm, okay, whatever.
However, then this article comes out from Reuters rebutting the above study. I think this could just confuse the public at large even more. I get really tired of using the BMI to gauge your perceived health. There are a lot more factors than weight and height that determine your lifespan. Lifestyle activity is such a big one, as well as what you eat. We really are what we eat.