The GPGnet Forums have word that there are no plans to implement DirectX 10 support in Supreme Commander:
"Despite the best efforts of our own team and our partners from Microsoft and Nvidia, the timing simply did not line up. The availability of DX10 hardware, final releases of Vista itself, and the solid drivers simply did not line up with the necessary time frames in our development cycle. Having missed that window, our dev team has focused on graphical improvements that will benefit a far wider audience than the still very small but growing segment of Vista users with DX10 compatible cards. Having missed that window, the resources necessary to integrate the tech do not make sense compared to what can be accomplished for that greateraudience."