All games out now are DX9, this card DX10. If Apple used a DX10 nVidia card, seeing they don't have a mobile card yet, we would still have the same problem.
Actually, they do, and it is in the MBP: The 8600M. But yeah, you are right that DX10-class cards are almost all improvements in the shaders, rather than fillrate or polygon draw rate. Unified shader architecture, more shaders and geometry shaders are the bywords of DX10-class cards.
Plus what is the use to have a DX10 capable card when you need Windows Vista for it anyways!?
DX10-class cards don't /need/ DX10 to work. They work just fine with DX9 (obviously)... and from the standpoint of the card, what level of DX they support only talks to what sort of minimum hardware is on the card. So to be DX10-class, a card must support shader model 4.0, and a couple other minor requirements.
DX10 cards will work with OpenGL 2 just fine, and I believe there is an updated OpenGL spec that supports the geometry shaders. You can tap into as much power of a DX10-class card with OGL 2 as you can with DX10. DX10's big changes are really on how software runs on the CPU, and how the programmer writes to the API. They make it Vista-only for a variety of reasons (which includes marketing and promoting Vista upgrades).