According to the Fight Club author:
Clark Gregg...knew that I'd written Choke while listening to Pablo Honey, with 'Creep', over and over and over. So Clark got Radiohead to contribute a song; to write a song for the very end of the movie, the final credits. Apparently Radiohead liked the movie so much, they've written the score, most of the ambient music throughout it. So it's 'Choke-- with the music of Radiohead'.
I've always thought that they would be really great at creating ambient scores for weird little movies like this. See There Will Be Blood for confirmation of their mad scoring skills. I was excited to see this before, but now it's even cooler.
UPDATE: Well, it appears that this is a load of bullshit. Pahalniuk was talking out of his ass, which I'm unsurprised about, and the score will NOT be done by the band. In fact, the song they contributed isn't even new, but is from their latest album In Rainbows, and is called Reckoner. It's an awesome song anyway.