Great line of dialog, Scott. Good way to end 2009.

:-)

It is THE gold standard of romantic comedies. I have yet to see anything since that has come close.

The most interesting part of this scene actually comes after the part you highlighted, though I must admit I only realized it when watching your clip on youtube.

Harry makes first move. In a classic feint to the negative, Sally rejects it. So Harry digs deeper, gets specific and hits the emotional bull's eye. She caps the moment with the "it's impossible to hate you" speech.
The scene could have ended there. Many a lesser writer would have been ecstatic to get that far. But Ephron has one more card to play. I call it the 'off-topic feint' -- a line that seems to draw us away from the immediate moment, or pull the characters away from each other and into the broader context of the scene -- in this case, a New Year's Ever party with Auld Lang Syne playing as the ball drops. On its own, without reference to the ultimate strategy, this is a brilliant tactic: Harry makes an 'off hand' comment that's typically clever and charming -- what the hell does this song mean, anyway? It sets up their reunion as a fait accompli. They can now return to 'business as usual' -- just talking about stuff and enjoying each other's company.
The great narrative coup come just after Sally's halting, heartfelt attempt to answer: "Maybe it means we're just supposed to remember we forgot them or something."

Ephron takes this seemingly random bit of chit-chat and uses it to pull us back into the absolute thematic center and the emotional heart of her story:
Sally says, "Anyway, it's about old friends."
Aint it the truth.
This is close to genius, and there's a lot to learn from it: using the apparently trivial external aspects of the scene to define and fulfill your story.

Whatever her later failures, you can't take this one away from Nora Ephron.
Happy New Year!