Soda Taxes and the Expansion of the Nanny State

Our government, it seems, is now mulling a tax on soda because of its link with obesity. Although this seems ridiculous at first glance, I can understand the rationale once a public health care system is in place. Once taxpayers are paying for health care for others, the government has some sort of implied mandate to limit costs by improving health. The soda tax may be a bit over the top, but how far can they take this idea? What can the government legitimately do to limit healthcare costs?