The DirectX 10 API will have completely new and faster dynamic link libraries (DLLs) and is supposed to run much faster. The company decided to cut the backward compatibility with DirectX 9, 8, 7 and lower in this API but there will be a way to use games programmed for those APIs. Microsoft will enable support for DX 9 or lower games through a software layer, meaning it might run slower.
http://www.theinquirer.net/?article=25902
...But it's still early anyone's guess what will be the actual case when DX10/Vista finally ships next year or after... also do note that most current PCs won't even be able to INSTALL the thing; and if you can't do that you can (have to) use XP and DX9.