The problem is that crap that's unhealthy is:

1. Generally cheaper.
2. Generally quicker and easier.
3. Tastes far better.

and the healthy stuff is:

1. Generally more expensive.
2. Quite often not quick or easy to prepare.
3. Tastes like CRAP!

The reason I gained weight after college was easily due to those 3 factors, plus I've always liked to eat "good" food. The ticket is to make the healthy stuff taste better, make it cheaper, and be something as simple as popping it in the microwave or oven and that's it. Neither my wife or I want to spend the time cooking crap when we're done with work, we're tired and nasty and hungry.

Taxing food is not the way to go and doesn't attack the core of the problem, IMO.