You can register or get from others . So far , it is not the problem for me . but it is against the policy of .us domain . Only the US resident can register or you must have business contract at US.
Yes you can buy it even you're not US resident. In fact,i do owned a few premium .us domain and develop it into some US based soccer fan forum.
To paraphrase Jerry Seinfeld in his rental-car agency episode "Anyone can take a reservation - the trick is to hold the reservation". http://www.youtube.com/watch?v=A7uvttu8ct0 Anyone can register a .US (no proof required). The trick is to hold it if they do an audit and ask for documents proving your eligibility.