Assassins Creed is not a 360 exclusive. It's gonna be released on both the 360 and PS3. Although Bioshock is still a 360 exclusive, there have been a lot of rumors that it'll be released on the PS3 as well, and the game maker hasn't denied it.
MS announced that the following games will be exclusive to the 360:
Assassins' Creed (Sony had thought they would have it too)
The next Splinter Cell (after DA)
And two exclusive episodes of GTA IV
Also, Halo Wars was announced as an RTS game based in the Halo universe and is being made by the same people who made Age of Empires 3. There is a trailer up for it so go check it out.
All this news definitely has me excited to be a 360 owner.