A reader writes:
Contra your speculation, my guess would be that Obama's strength in NYC comes mainly from Brooklyn, not Manhattan. Brooklyn = Minorities + Overeducated White Youth; Manhattan = Rich Old People + Tourists + Harlem.
All of the black politicos in Brooklyn are lining up behind Obama; not so the Harlem powers-that-be. Everywhere you go in brownstone Brooklyn you seem Obama stencils and posters; I'd be really surprised if the closeness of the NYC race wasn't emerging from this dynamic.