The NDA for coverage on the PC version of Assassin's Creed has obviously been lifted. Stories pop up all around the world, but benchmarks of video cards and processors are rare. German site PCGH.de shows comparison benchmarks of multiple hardware setups, including DX9 versus DX10 under Windows Vista.
Interestingly enough, DX10 is much faster than DX9 when using low details. That is due to shader model 4 of DX10 that is very efficient in sending a lot of data to the graphics card. But if you crank up the system with max detail and anti-aliasing/anisotropic filtering, DX10 is somehow slower than DX9, especially on Radeons.