I can see why Lee won't be converting to GL, seen how much code goes into a spinning cube.
If you want to develop mac, use Ogre, irrlicht (as said) or Torque as SDK's or use Unity (Requires a Mac but can export to PC depending on the version) or BlitzMax, there are 3D packages for it, or you could openly code open gl with it
Or just development with C#, you could use 3D State's SDK, that has C# specific support as well as other programming languages
or Truevision SDK, which will be releasing its new update soon (which will be a lot more powerful)
Kewl looking SDK's are Ogre and TV3D, but Ogre seems to kill my FPS rate