The number of American citizens and green-card holders severing their ties with the U.S. soared in the latter part of 2009, amid looming U.S. tax increases and a more aggressive posture by the Internal Revenue Service towards Americans living overseas. According to public records, just over 500 people worldwide renounced U.S. citizenship or permanent residency in the fourth quarter of 2009, the most recent period for which data are available. That is more people than have cut ties with the U.S. during all of 2007, and more than double the total expatriations in 2008. An Ohio-born entrepreneur, now based in Switzerland, told Dow...