The Tax Foundation has weighed in against a "fat tax" (previously blogged here, here, and here), with a post entitled Swatting the Obesity Fly with the Tax Sledgehammer:

One problem with obesity taxes: they’re an extraordinarily blunt tool for social policy. A fast food tax, for example, would penalize fit people who enjoy an occasional burger as well as the obese. If the true goal were weight reduction, why not take more direct steps toward that—for example, by putting taxpayers on a scale upon driver’s license renewal and assessing a "weight penalty"? As we’ve written before, the more we ask of the tax system, the more it asks of us. Implementing social policy through the tax code—rather than direct spending programs—leaves the rent-seeking barn door wide open for tax-preferences-seekers of all persuasions.