This is an interesing article from Wired Magazine dealing with seasteading.
I have heard about this once or twice. Apparently, some libertarians believe that one might establish a "stateless" society outside of a nation's territorial waters (apparently 200 miles). I wonder how many people it would take for such a mini city to constitute a nation for purposes of the UN membership (not that most libertarian nationists would want to become "member states" of the UN).
According to the article, the first such colony (or primitive version thereof) was created by an admirer of Ayn Rand, one Werner Stiefel.