XNA is something different.  It's going to be a higher level set of API's used to cut down on boilerplate code in future games.  It may use DX10, but it isn't DX10.

Also, I very much doubt that DX11 will be out before Longhorn.  DX9 was designed to be a reatively long lasting api compared to previous versions, and we still dont have a retail card that supports pixel shader model 3.0

DX10 would no doubt contain several more 'profiles' that will take another 2-3 years for the hardware to catch up.

When it comes to DirectX, the software is definately outpacing the hardware.