I just saw a commercial on tv in Holland which talked about an X06 contest. It said: Play these games before everyone else. Assassins Creed was among those titles. So it will be playable at X06, maybe obvious anyway but I just wanted to point this out.
The contest can be found here:
http://msn.xboxchallenge.nl/contest/
On the page it says:
Already play or experience the games that will hit stores this fall, like Splinter Cell: Double Agent, Forza Motorsport 2, Pro Evolution Soccer 6, Call of Duty 3, Need For Speed: Carbon, Assassin’s Creed or Gears of War.