There is going to be an XBox360 version of the game, so it must support DirectX 9. That doesn't preclude the PC version from supporting some DirectX 10 features, but I doubt Larian had enough lead time to build DirectX 10 in from the start, or the resources to overhaul everything afterwards. Also, DirectX 10 is Vista only, and I don't think Larian would want to limit their target audience (depending on when the game will be released), since RPGs are already a niche market.