To elaborate the above post:

You can use ATi (regardless of Crossfire capabilities) or nVidia (regardless of SLI capabilities) on nVidia or ATi platforms without issue. If you want to use Crossfire or SLI, then limits kick in:

Crossfire: ATi or Intel chipsets
SLI: nVidia chipsets, ULi chipsets (unofficially)

To further dismiss any rumors and such, it is not a hardware limitation, but a driver one. nVidia and ATi have limited, via drivers, what platforms their multi-GPU technologies can be implemented. There's a few drivers floating around that can do that, including hacked ones, but I don't have any links.