CDP has said this is primarily due to want to put down more polish on the game, but in reality it's far more likely that this is a result of the currently legal dance CDP is locked in with Bandai Namco over who gets to publish the thing and why. Since there's probably an injunction on the release until this thing gets hashed out, CD Projekt might as well take the time to optimize and polish the 360 version up, right?
Right.
CD Projekt boss Adam Kicinski had this to say "“The extra time will allow us to polish the title to perfection, so that the game offers an ideally balanced, fantastic experience to gamers. As the launch of the game approaches, we will be sharing more details how it will be released on Xbox 360. I trust that despite the release date shift, players will be satisfied with what we are planning for them.”
CD Projekt will apparently also be showing off the "major DLC" that will be coming for the PC version at the upcoming GamesCom event later this year.
via vg247