The problem is that DirectX 10 video cards don't exist yet. Nvidia has the G80 in the works and ATI is polishing up the R600, a GPU based on the Xbox 360 "Xenos" core, but cards won't appear until the second half of this year. Even the game companies are having a tough time getting early-development GPU hardware right now. Several game developers we've spoken to have told GameSpot that they're still waiting to see DX10 reference cards.
Aside from a handful of Windows Vista DX10 launch games, most PC games shipping in the next year will be DirectX 9 titles. Since the number of Windows XP DX9 video card system owners will dwarf the Windows Vista DX10 video card installation base for quite some time, we'd have to guess that the DX10 game developers will create DX9-compatible fallback versions of their games to make them playable on current hardware