This story occurred on Dec 17, 2007, but I haven't heard about it from any mainstream news organization. I am posting it here, in the hopes that someone will provide some additional information (in the comments) about the truth of the story.
According to the story, several 'representatives' of the Lakota Nation declared that they are seceding from all treaties with the United States and Canada. Upon further research on the web, I learned that the group making the statement is called Lakota Freedom and the leader of this group is Russell Means. There seems to be some discussion about the representation of Russell and Lakota Freedom for the larger Lakota Nation.
It's also interesting, I tried to goto this link today: http://www.lakotafreedom.com/
But I got a message in my browser saying that "HTTP: Bad Request"... Seems like the site has been pulled down for some reason.
I'd like to learn more about this story. It's an interesting idea, seceding from the United States. One which I think the indigenous nations are entitled to if their treaties are bogus, but something that is likely to garner a interesting response from the president and the nation if indeed it's true.