I believe the wording below is a bit restrictive - from my award letter:
"Fellowships are funded for a maximum of three years and may be used in any three, 12-month units, starting in Summer (June 1) or Fall (September 1) over a five-year period that begins in 2012 (your award year).
That means if you accept, you have to take at least one year of funding immediately. Which means you can't defer all of it immediately, and if you're allowed to take multiple fellowships NSF must come first for a year, at least, in order to keep it. My award letter doesn't say anything about accepting other fellowships, but there may be other sources in the NSF website that do.