Is there any reason why, with the same code, everything would work fine on os 9 with my carbon opengl test http://david.emuscene.com/Terrain.jpg but on os X it runs exactly the same except there are black lines at the edges of the skybox and depth testing refuses to be enabled?

I don't know about the depth testing issue (are you requesting a depth buffer?), but the black lines around the edges are most likely due to you using GL_CLAMP for the texture wrap mode, rather than GL_CLAMP_TO_EDGE(_SGIS) (core in 1.2). Virtually every OpenGL vendor in history had mis-implemented GL_CLAMP as GL_CLAMP_TO_EDGE. Apple fixed their implementation on Mac OS X, which caused a few existing programs to break.

Ok thanks I'll try using GL_CLAMP_TO_EDGE. About the depth buffer... I just started using carbon so I'm not totally clear on how it works... how do you request a depth buffer?

It's just the same old AGL as you're used to on OS 9. In your pixel format attribute list you'll have something like:

AGL_DEPTH_SIZE, 16

Thanks a lot, it works now