Thanks! I submitted a bug to the developers regarding this.   Is there a workaround to make it report 10 instead of 9Ex until then?

In DxDiag, the General tab reports DirectX10. On the Display tab, it reports DDI Version: 9Ex. This is using the latest nVidia 64-bit Vista reference drivers and August 2008 DirectX redistributable in Windows Vista Business x64. Microsoft Support says that it's a known issue but it doesn't affect performance: ...