According to a Kotaku interview with Rockstar, the 360 version of L.A. Noire is going to require three separate disks. This does not mean that you will be forced to get up and switch your game disks every two minutes, though. The way they have structured the game, you will only have two points in which you well be required to put in the next disk to progress the story.
While the three disk requirement is partially a testament to the amount of content within the game, most of the extra space is probably for the new facial animations, and other technology Rockstar has included to make the game feel more realistic. All of those new animations and ways of immersing you in the game can take a ton of space on a disk, so it should be no surprise that they needed three to get the game out. PlayStation owners, however, will only have one Blu-Ray disk to worry about when playing L.A. Noire when it is released May 17th.