Ok, i read, unreliable rumour from someone who knows someone who works at MS. (Yeah rediculous, but no better than the info been currently thrown around). Anyway, apparently how its going to work. You can buy you xbox game, the disk won't actually work on your Windows 8 machine. You will just get a license to download a pc version of the game (Since xbox live and windows live are merging). But heres the kicker, to play say the windows version of the game, your xbox will have to be online as well, so MS can check you are not playing the xbox version. You are not going to be allowed to play both versions at the same time.
