It may be that some time in the future, His Holiness will travel to the USA. The terms are still quite vague, however:
VATICAN CITY, APR 27, 2007 (VIS) – Yesterday afternoon, Holy See Press Office Director Fr. Federico Lombardi S.J. announced that Benedict XVI has accepted the invitation presented recently by Ban Ki-moon, secretary general of the United Nations, to visit the headquarters of that organization in New York.
"The Pope," said Fr. Lombardi, "has accepted the invitation in general terms, and has expressed his willingness to visit the U.N. headquarters, although as yet there is no date or program for the trip."
Servant of God John Paul II visited the U.N. headquarters in 1979, and again in 1995 for the occasion of the fiftieth anniversary of the foundation of the organization.