Ok. I have NSF. I read this very carefully too before I got it(was considering delaying grad school for a year).
Fact is:
1) You can ONLY defer for a year if you are a) a full time studuent getting other funding at the time, b)military service, or c) some special special situation, like family problems or something. They will not let you frolic the world, take a job, do something else, etc with your deferment. You can do this deferment for 2 years during a 5 year period. I am currently deferring because I'm on a "first year" PHD fellowship at my school, so I figured why not take the money?
2) You most likely will be able to beg a school(not ANY school, however) to take you. Some schools frankly don't care if you have your own funding. Obviously, it helps with regards to "keeping" you after you are trying to pass qualifying...why would they fail out a free student that shows a small amount of potential, but in my experience, it doesn't help you to get in to certain schools. They have their own criteria, and may not have simple things like space to have you there. So feel free to ask, but it will be ok.
3) If you get the NSF, it means you are a kickass writer(on some level). This means that you will write decent personal statements. So you'll get in somewhere (99% confidence).
Moral of the story: if you get NSF and don't go to grad school, you, by default, forfeit the NSF. Even if you intend to go within one year. This is clearly stated on the fastweb FAQ section.