Nick and Norah's Infinite Playlist by Rachel Cohn and David Levithan
And guess what, it is ten kinds of awesome.
They take turns writing chapters, and the whole thing takes place in one night, where two high-school age club kids in New York meet and fall for each other while listening to agressively hip punk bands.
Oh yes, and they meet because Nick is trying to make his old girlfriend jealous, so he asks Norah to be his GF for five minutes, and she thinks it's ridiculous but they kiss...and oh my god, is it true love?
Well, unfortunately my advance copy was missing every other one of the last ten pages, so I'm not sure, but yes, I think it is.
And of course, bonus points for the Thin Man reference.
Oh, and speaking of the Thin Man, did you know that the character of Nora Charles was supposedly based on Lillian Hellman?