This is the more general project that has emerged from my initial idea for Darwine, namely X86/Linux binary compatibility for OS X.
Hmm, what is that useful for? Nearly all software for Linux is open source and can be ported or sometimes simply recompiled for any given platform. Do you have any particular programs in mind for this ability, or is it just for the cool factor?
Doing a binary emulation of Linux is really, really easy compared to emulating Windows (or MacOS) because all the supporting libraries are already open source. FreeBSD does it, for instance. You only have to do the syscalls and maybe the linker/C library.
IIRC there is also a "Line" program that does Linux binary emulation for Windows though for obvious reasons it was never used much ...