Number of people in the US via 2000 census: 295,589,508
http://www.census.gov/main/www/popclock.html

Number of DS units sold as of the end of december: 1,300,000
http://www.eurogamer.net/article.php?article_id=57545

295,589,508 / 1,300,000 = 227.3765446

Take into consideration some mild population growth since 2000 ~_~, and since you cant have a fraction of a living human, round up to 228.