it's much easier to port from Un*x to OSX
than vice versa. A far greater number of
*nix libraries have been ported from the *nix world to OSX than the
other way around. You can even run X windows
on OSX
I think "OroboroSX", or some such name, is an X window manager
for OSX that makes X apps fit in with Mac apps so it's not
obvious which style is running. Gtk and Motif have been ported,
so the GUI is not a big issue. The audio side is still primitive,
but easy to use. Porting Snd took a few days, mostly spent
chasing down dumb things like the -framework loader switch.
I'd say it took about the same effort overall as porting
to the Sun.