Jonah Lehrer is all for a soda tax:
Just look at cigarettes: If you want to decrease the numbers of smokers, raising the price of cigarettes is the only proven solution. In fact, a 10 percent increase in the price of cigarettes causes a 4 percent reduction in demand. Teenagers are especially sensitive to these price changes: a 10 percent increase in price causes a 12 percent drop in teenage smoking... And let's not forget that nicotine is an extremely addictive substance. (While high fructose corn syrup might taste good, it's not chemically addictive.) So I think there's good reason to think that a 10 percent hike in the price of sodas might be even more effective than a cigarette tax.