The problem is that the biggest part of the port isn't in the basic platform code, it's in the GUI.

Yes, Mac OS X is a BSD-based UNIX, and Poser could in theory use straight UNIX coding for file I/O, memory management, and all the other utility functions. However, to run on Linux or FreeBSD (or even a straight Darwin) they'd need to build their GUI around an X11 model; while to run on Mac OS X they'd be using Quartz (and ideally OpenGL, but from what we've heard they're not initially doing real 3D screen output).

[You can run X11 on Mac OS X, but it looks really poor next to native Aqua applications, and it'd be foolish for any commercial Mac product to go that way. It does make sense for free Open Source applications, in some cases, but only because it's easier to port the code that way and in most cases nobody's going to invest the time and money to do it right.]

Furthermore, going "UNIX" would mean abandoning support for Mac OS 9 -- or supporting yet another substantially different code base to do it differently.

So far, the writing on the wall seems to be that the Mac Poser (if it ever really happens) will be Carbon, not Cocoa or UNIX -- that is, it'll use the improved versions of the traditional Mac toolbox that can be made to work both on Mac OS 9 and Mac OS X. A Carbon application cannot be made to work on Darwin, Linux, *BSD, Sun, etc. It's Mac-specific just as the Windows version is Windows-specific.

They could certainly do a third UNIX/X11 version, if there's enough demand. If they stick with POSIX and portable X11 interfaces, they should be able to build it for just about any UNIX. It's not a trivial job, though; and adding extra UNIX versions isn't just about recompiling... the big cost is in testing and deployment.