There are three possibilities for running OSX applications on Linux;
1) Wine-like emulator (bad for the same reasons as Wine is bad... nothing works.)
2) Virtual machine (way too much overhead, zero integration...)
3) Take existing workable OSX userland and run it on a Linux kernel.
#3 is what I'm talking about. A similar thing is done for running debian/GNU userland on Android devices: http://www.mayrhofer.eu.org/debian-on-android
... of course the advantage there, is that there is no problem interfacing that userland with the Linux kernel.
Basically, we don't want to have to run a second kernel, and we don't want to be stuck with the hackish wine-type crap where everything has to be reimplemented the hard way.