The Xbox 360 version will be an enhanced adaptation of the PC game, which was released in mid-May. Though finer details are yet to be confirmed, Atari says that this new version will incorporate numerous improvements and enhancements. Further details will be announced in the coming months.
Incidentally, if you're wondering what's happened to our review of The Witcher 2: Assassins of Kings, then it is coming. Unfortunately, the game was lost in transit so we've had to wait for another copy. We'll have a full review for you shortly.
Here's the official E3 trailer...