I'm in favour of using SOABI on OSX as well.

The build system currently uses "darwin" for the platform triplet, that's a bit to minimal.  I'd prefer to use "macosx" instead of "darwin" for the platform name, the same as is done for platform specific directories/files in distutils.

I'm not sure yet about what to do with the hardware architecture and deployment target.  On the one hand it could be nice to reflect those in the name of extensions, on the other hand that makes it harder to mix and match binaries.

In particular, a fat binary for Python with deployment target 10.5 can currently load single architecture extensions with a different deployment target. That is useful functionality to have (for example to be able to load C bindings for newer OSX libraries in a python.org binary, the newer OSX libraries don't support PPC and some of them even don't support 32-bit i386 code). It would be a shame when the extension name would have lie about what's actually inside.

BTW. It might be useful change the suffix for extensions as well when adding the SOABI. The extension is currently ".so" and might be changed to something like ".pyext" to make it clearer what the file is for.  That would also make it possible to use custom icons for extensions in the Finder and be smarter about them in spotlight and quicklook.