Yes, nonviolence has proven to be an effective means for political and social change in the United States. In fact, I can't quite think of a violent way to produce political and social change; nonviolence is really the only way to get things done is modern societies. It can be frustrating, especially when one's opponent resorts to violence, but most progressive movements over the past 100 years have proven that nonviolence works.
Sadly, America is built on a history of violence, genocide and slavery. The myth of rugged individualism is still a driving force in American society, and eclipses the ideals of compassion and community-mindedness. There's a growing class divide, and increasing poverty will likely result in more violence, not less. I won't lose hope, but for now I think violence is here to stay in the US.
With the political polarization of the religious right, ironically, so much hatred has become so visible in America, that I don't believe American nonviolence is possible. With the election of a black president, prejudice is alive and well and there exists so much vitriol and prejudice in this country that it seems people are more and more resorting to violence in order to be "heard".
Unfortunately, I do not think that American nonviolence will ever be achieved or is even possible since too many people want to hurt each other. There are terror groups inside the United States of whom want to kill others just because they do not believe what they believe, which is why peace cannot happen.