PCGH made an interview with Charles Beauchemin, Technical Lead of Assassin's Creed PC.
PCGH: When porting for the PC have you thought about integrating support for DirectX 10 into the engine? Will there be a version of Assassin's Creed that will utilize the new Vista API?
Charles Beauchemin: Yes, Assassin's Creed supports DX10 as well as DX9.
PCGH: What are the technical advantages of that API?
Charles Beauchemin: DirectX 10 enables us to make fewer calls to the API to perform the same actions. Therefore, it will be possible for the drivers to make optimizations making the game faster without any change to the game engine. Since most of the DX10 drivers are still young, we can expect a lot of gain to come from the optimization of these drivers.