The Freakonomics blog has an interesting post on The Economics of Obesity: A Q&A With the Author of The Fattening of America. 

Basically, the financial costs of being overweight are getting lower and lower as our society [the U.S. in particular] makes significant advances in medicine, specifically drugs. Our own technological advances are creating less costly economic options that in turn give us less incentive to be physically fit.

Here's a personal example.... I have a small refrigerator in my office so I can keep cold water, my lunch, yogurt and beer [yes, that's allowed at MSFT ] close by instead of having to walk half way across my building to the 'common' refrigerator. I bought the little fridge cheap as it was a scratch-n-dent display model for sale. Sure, it would help me burn some extra calories to use the 'community' fridge, but mine is very convenient, I don't have to worry about other people taking my stuff or leaving it too long to be thrown away by the cleaning crew.

But here's another example of the flip side.... I was diagnosed with slightly high cholesterol last year and my doctor said I have two choices. Take a pill [Lipitor anyone?] or change my diet/exercise. For me it was pretty obvious what to do based on my lifestyle [I already go the gym regularly and eat fairly well], but consider that person for whom diet and exercise would be a significant lifestyle change. If they have good medical insurance and no intention of exercising then the pill could be the obvious [and most economic] choice.

~tod