It's not really any different from running DX7 games on a DX9 installation.
The calls are different so there's another entry into the HAL that's used.
I doubt another software layer needs to be added, and if it is I doubt you'd notice the difference unless your system is already marginal at playing the game in question as the added calls would be CPU calls rather than GPU calls and most games nowadays are far heavier on the GPU than they are on the CPU.
As an example: I noticed no performance improvement on most of my games when switching from a 1.6 to a 2.4 GHz CPU while keeping the same videocard, only when upgrading that as well did I see a marginal improvement and that mainly in display quality rather than speed.
The whole thing sounds to me like a prelude to another round of Microsoft bashing, preparing for the claim that "Microsoft is trying to drive game manufacturers out of business by preventing their games from running on Vista".
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duckman
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