The Times had an article about this in the Week in Review a couple of weekends ago (which you can read at http://www.nytimes.com/2010/02/14/weeki
Taxing soft drinks as a way of dealing with the obesity and diabetes epidemics raises the prospect of needlessly prohibitive pricing for one of life's few remaining reasonably-priced innocent pleasures.
As someone who has always been slim despite regular consumption of Coca-Cola, Dr. Brown's Black Cherry and Cel-Ray, 7-Up, and other icons of America's soda-pop heritage, I don't see why I should be taxed because other people are eating (or drinking) too much and getting fat.
Furthermore, the article makes seven references to these beverages being sweetened with sugar. In reality, almost none use "sugar" in the everyday sense of the word (i.e. sucrose from sugar cane), but in fact are actually sweetened with fructose from corn.
If we're going to tax anything in an attempt to control our weight, let's tax fructose and corn syrup not just in soft drinks, but in ALL their myriad unnecessary applications in our industrially prepared foods. (I just saw yesterday that my otherwise healthy favorite herring snacks now get their modest sweetening from high fructose corn syrup, which is ridiculous.) At least that would affect everyone equally and would begin to make up for decades of politically-maintained price distortion that has kept our fructose cheaper and our real sugar more expensive than anywhere else in the world.