MS said a while back that it's supposedly not being released on the PC Platform, after 6 years of development, even though that the game has been already fully developed for the PC and ready for release.
Remedy have completed the game, But its up to Microsoft Game Studios, the publisher to determine what platforms it sells on.
They've obiously chosen the Xbox360 right now for the first release to be $$$ Cows.
I believe once they realise that theres money to be made from releasing it to PC, it will be out and published to us. Nobody has any idea how long this will be though.