Fairly easy fake...
If you goto EBGames or Gamestop and look at their "Coming Soon" wall, you'll see placeholder boxes for GoW. They look JUST like that. This person just cut one apart and stuck it in a 360 case. The CD is fairly impressive if it is a fake, but not insurmountable. A good photoshop hand and a color printer will yield similar results.
The real giveaway would be the back of the box. If there is detailed info and ratings, etc. then I'll believe it. Sorry to be skeptical, but I highly doubt box art would be finished 5 months before release.