It makes more sense to me to tax the people who are adversely afected by the junk food. My high metabolic rate, combined with my skinny parentage allow me to eat as much junk as I wish. Therefore, I personally would not be effected by a tax on obese people.
Another idea would be to have people weigh in each time they entered a fast food restaurant. If they exceeded the weight limitation, they would only be allowed to order from the low-cal menu.
We could also start checking ID's when a consumer attempts to buy a high fat snack. In this way, we would restrict children from buying these dangerous products.
By combining the recent tobacco settlements with restrictions such as these, we would soon be able to boast of a smoke-free, obese-free country. Our beloved politicians would then be free to further fine-tune our lives, shaping them and molding them ever closer to perfection.