XP could technically only support 3 GPU's (not in reality I believe?). I believe Vista's limit is 4 GPU's.
2 GPU's are easier then 3 or 4 GPU's.
So 2 GPU's are not that big of a problem, but what about 4 to 8 GPU's? I bet it gives much more problems then the standard 2 GPU SLI or Crossfire rig.
I think this is a good choice from M$. It's better they work together with ATI and nVidia to solve the current problems and release the support in a service-pack.