So is it a hoax? That's the first thing which crosses my mind. Most news articles are clearly rehashes of the main article and since I've not seen anything official from the US government about it. This month's State Department Magazine (pdf), posted yesterday, mentions nothing about this, but since those are presumably prepared well in advance, that's not surprising. The State Department Web site doesn't mention this, even to deny it, so I'm a bit suspicious, but still, it's interesting news.
What does this mean? Well, who knows? The US is in a pretty poor position to do much about it now, but the US has allowed people to homestead on Lakota land and that alone could be a thorny political problem. Allegedly, the new Lakota state will start putting liens on people's home within five years if the US does not immediately engage in negotiations over the secession. Property rights alone could make this a nasty dispute. They also apparently plan to issue their own passports and driver's licenses.
If this is true, it could potentially embolden quite a few other native American nations. If the Navajo nation were to follow suit, they have large areas of Arizona, New Mexico and Utah and about 1/3 of a million people live there. I doubt there would be a huge immediate economic impact, but the political impact would be enormous. How many other Native American groups would want to secede? And would others consider following suit? Many states in the US already have secession movements, albeit small ones. The U.S. map could look like Swiss cheese. Somehow, I doubt this will come to pass.