Here is Nakamura's annotation of the Gelfand-Nakamura game.
http://main.uschess.org/content/view/10023/571/
He admits he was influenced by Charbonneua's game from Montreal that featured the exact same sacrifice. BTW Charbonneu has said on CLO that his game was all home preparation.
I'm surprised the Chessbase annotator didn't mention that game and said this about Naka's Nxg2 "Double exclam for this beautifully unexpected move, and for the courage to play this against the world's number six player, who is 53 rating points above the American."
Naka didn't even give it one exclam and says it leads to a draw if White plays Kxg2.
BTW as far as the opening, they were following the Belieavsky-Nakamura game up to move 16 when Gelfand deviated with 16.b5 instead of 16.a5 as was played unsuccessfully by Beliavsky. So it seems as though Gelfand did come prepared, his 20.d6 being the new move, but alas he didn't go far enough.