Question about SM 4.0, DX10 hardware, & OpenGL

MS decided to force (yes... this is more, or less a "strong
arm" tactic) gamers to give up games that require earlier than DirectX
9 on their new Vista machines (how much of a hit does DX9 games take
since DX9 is emulated in Vista?). They also made sure that if we want
to play new DirectX 10 required games, that we'd have to drop WinXP,
instead of coding DX10 for WinXP. This only shows how Vista is to
WinXP as Win98SE was to Win98FE... A few added features, but
functunally the same.
While non-gamers have Linux, or other open source OSes to go
with, us gamers are stuck with MS. Since most game developers seem to
only support Windows, & DirectX API, what if all games supported
OpenGL as well? Would this allow new games that uses new SM 4.0, &
such to work under WinXP? I'm not up to speed on how OpenGL works, or
if it can use the hardware effects that DX10 uses to get that new
eye-candy. If it can produce the same effects as DX10 using Nvidia's
8xxx, or ATI's R6xx GPUs, can it work under WinXP, or is Vista still
needed?
I'm not so ****ed that MS is moving to Vista, but I'm ****ed
that I have to keep my current system for WinXP, DX9, and lower games
while building a complete new system for Vista instead of just
upgrading it with a DX10 videocard, & still being able play all the
games that work in WinXP (they could have at least made DX10 backwards
compatable like DX9 is, or at least made DX10 for WinXP).