Can't see a link?
http://www.theinquirer.net/?article=25902 If it represents a significant break in the backward compatibility, a new name, rather than "DirectX 10", would have been better.
Interesting to see how the permutation pan out, as regard software layers and compatibility.
Since it would be probable that it becomes available while the majority of software and hardware is still DX9 or lower, and people will probably upgrade without reading in full, the case of DX9 card / DX9 driver / DX9 software must be well catered for.
For producers to USE a new DX10 API, it will also need to have decent compatibility with their minimum spec. or their market sector would be limited to those who purchased DX10 hardware.
Obviously, the ideal will be new DX10 card, DX10 driver, DX10 software.
Those running existing games on new hardware will want good performance from DX10 card, DX10 driver, DX9 software.
Those running new games on existing hardware will be interested in - DX9 card, DX9/10 driver, DX10 software.
To treat backward compatibilty as an afterthought, seems to be an ill-considered move.
Actually, with the unified drivers of both ATI and Nvidia, it would seem that the old cards will very likely get DX10 compliant drivers, so long as the lack of hardware capabilities can be allowed for - eg. a Geforce 2 can be currently used under DX7 card / DX9 driver conditions, though WHAT it can run is getting limited - for modern games, it seems that DX8 would be a more practical minimum target.