I don't see the point. The only special issues to discuss are hardware/software compatibility and performance, and the occasional platform-specific features such as keyboard shortcuts, mod-related software etc. The whole objective is for the software to be as compatible as possible with the Windows versions, so there should be no point in having separate modding, SGs, competitions and so on. Those would all come from the existing forums. And Boot Camp is going to blur the edges between the platforms even further.
A separate forum for Mac Civ4? Yes - watch this space.
A separate website with its own management and support resources? I don't think so.