No, DX10 will support DX9 cards.
DX10 cards just means that it has hardware chips on the card that can be used for shading, pixels, textures, or whatever the function requires (instead of hard-coded pixel shaders and vertex pipelines, etc, that you find in DX9 cards).
DX10 shouldn't have problems with DX9 cards.
That is to say, no more than the whole crappy shebang has with DX10 cards
