Actually, I think legislation could make a difference, but not legislation of the kind people bannter about -- closing McDonald's that are in proximity to schools and the like. That's useless.

Legislation that I could see as useful:

1. Revamp farm subsidies that artificially reduce the price of grains. The entire subsidies mess is one of the primary root causes in my opinion.
2. Force changes in dietary recommendations that clearly are not working.
3. Require better labeling of nutritional content in stores and restaurants
4. Remove all nutritional based marketing. Give us clear, concise, accurate nurtitional values, but don't allow claims of low-carb, low-fat, lite, whole-grain, organic or whatever. It's all gone. Right now, you can put a 'healthy' sheen on Fruit Loops by claiming they are trans fat free, organic and whole grain. Ridiculous. No way to fix that system - just junk it.
5. Treat sugar, when marketed to children, the same as you treat tobacco. No cartoon characters. No television commercials. Just more or less disallow food marketing aimed at anyone younger than 18.
6. Pay for physical education in schools. Require and pay for nutritional information. (I know, right now they'd teach the wrong lessons, but point 2 would hopefully help with that. At the very least, both viewpoints could be required).

Now, I wouldn't expect most of these to ever happen. Big Food is very happy with the current situation.