Strikes me they already know the date...from the New York Times in an otherwise ugly story:
“Practically the first thing Cardinal Egan did after Pope Benedict was elected was to invite him to New York,” Mr. Zwilling said, referring to Cardinal Edward M. Egan. “We had been hoping he would be willing to come. We first got word of this several months ago that the pope had accepted the invitation of Ban Ki-moon to address the United Nations. A few days later the Holy See made it clear it would not be in 2007, so from that point on we anticipated it would be in 2008.” Mr. Zwilling added that the visit to New York could occur as early as the spring, but cautioned that no firm date had been set.