Is a grassroots movement enough to change the conventional food system?

Our conventional industrialized US food system is at the core of pressing issues such as food access/security, obesity, and widespread pollution. It is simply unsustainable.
The locavore movement is an attempt to transform our current food system by promoting urban farming for the most part. Here are a few examples:

• Detroit, which has lost a third of its population, could be home to 10,000 acres of arable land in deserted urban neighborhoods, and create up to 35,000 jobs connected to the food economy.
• West Oakland CA lost access to fresh food as white residents moved out and the neighborhood became predominantly black; as a result, the rate of child obesity rose. Today, there is urban gardening, a food coop, and farmers markets thanks to grassroot efforts.
• Re-think, a New Orleans group led by engaged students, has vowed to change food in public schools. As a result of their efforts, school gardens have been established and sustainable practices have been introduced in the kitchens and cafeterias.

Food is political and our food system clearly reflects our values as a society. Shifting our entire system towards a more sustainable model entails profound changes in our society. While all these projects are encouraging, I think they are almost anecdotal. Change from the bottom-up might take a very long time to reach a critical mass. But we are facing pressing issues such as food security and obesity that, it seems to me, require immediate attention and action. I don't believe that regulatory changes at the federal level would be effective, but perhaps at the state level where there could be less partisanship and more focus on problems to solve. I am thinking about the Farm-to-Plate initiative in Vermont as a model to drive change. Maybe I am a bit naive though...

*Post is a response to Going Locavore show*