That, and there are probably limitations in DX9 that won't allow eyefinity to work like it should. "Eyefinity" is just what ATi calls their solution.
I'm sure every company that (eventually) makes a DX11 card for Win7 will have a similar (if not the same) solution that does exactly the same. AFAIK it's an inherent feature in DX11.
ATi being the first to the market was just able to call it whatever they wanted.
There was a huge fuss about the loss of nVidia span modes when the archetecture changed to DX10/11 from DX9. It's most likely the same reason why EyeFinity will never be on XP. The way things work in DX are too different between the guts of DX9 and DX10/11. Also the reason why XP never got DX10/11. AMD has essentially taken a feature nVidia discarded when Vista came out and greatly improved it in EyeFinity. One company's loss is another's gain I guess.