According to the March 8, 2004 edition of Time Magazine, "a single
[kiss] is [an] acceptable [greeting] in the United States, but it's
mostly a big-city phenomenon
The Wall Street Journal points out how it's affecting business, and when to do this.
Chicago Tribune has a piece on how it's spreading, and notes that it's not just a case of "is it in America now?" but that it's cultural, and different regions make sounds when they do it, might do one kiss or two, and it varies even in the same city.
Having traveled around there, I can confirm that one of my friends did it but perhaps because she has a lot of Latin American friends, and she points out that some of her friends were taken aback when she did it in Florida. So short answer - it depends. I'd watch the person you're greeting - if they go in for it, follow their lead, until you feel comfortable with what to expect.