@Rodney
Please read the GameSpy article again. Cg will work on top of DirextX and OpenGL, and although Microsoft are mentioned, nVidia make it clear that they want to make Cg a cross-platform standard both in terms of software and hardware platforms.
The real question is whether the OpenGL compatibility of AmigaOS 4.0 will be up to scratch so that Cg would work on top of it.
Note also that Cg will be open source, which bodes well for portability/