Assassin's Creed 'white screen of death'; possible fixes
We know that firmware 2.01 is coming along soon and there is a possibility that this will include a fix for Assassin's Creed.
In the meantime there is a suggestion that if you're running 2.00 you only need to disable the new Information Bar to stop AC from freezing. It has also been suggested that signing out of the PSN before starting the game helps as well.
So if you're having issues, and since there's no on-line multiplayer element in AC, you may as well sign out of the PSN before playing to ensure a seamless experience until 2.01 comes out.