“Half Life Counterstrike will not completely start in Direct3D mode. To avoid this, contact your video card vendor to obtain the appropriate OpenGL driver and run the game in OpenGL mode.”
Well, DX10 is vista only, but that doesn’t mean DX1-DX9 won’t work. All your old DirectX titles should work in Vista, but obviously things are still in beta so it’s not surprising if some apps break. I would expect this to be fixed before Vista is released.
DX10 will support DX7 to 9. Anything earlier is killed. DX7 to 9 will run through a emulation layer that will put that fixed function code through DX10’s shader only.
That’s actually what GL drivers must be doing on some GPUs.
The WINE project (and it’s commercial rip-off cedega) try to write a DX9->OGL wrapper for some years now, and it works only for some very popular games, but not in the general case.