He came to Washington, D.C. and New York in 2008. There are no announced plans (at least, as far as I've heard) for another visit.
By way of reference, John Paul II came to the US five times during his pontificate (excluding two stop-overs in Alaska): 1979, 1987, 1993, 1995, and 1999. But he was much younger: in 1979, he was only 59 years old, and he was still under 80 by his last trip here. Benedict is already 85.