The project aims to run OS X binaries on Linux in a seamless manner by leveraging GNUstep for the Apple Cocoa frameworks and implementing other OS X components needed for offering binary compatibility. The project is similar in nature to Wine but rather than running Windows applications it is about OS X support.
I first covered the project in early December of last year when it was still in its infancy; A New Project To Run Mac OS X Binaries On Linux. I hadn't heard of Darling since then, but the project is still fledging.
The Wiki for the open-source project reflects semi-frequent activity, including the documenting of BSD syscall coverage, missing GNUstep classes, OS X installers, the core foundation, building the code, and other changes.
While moving ahead, it seems the application compatibility for Darling is still quite small. While not much is implemented yet as it concerns end-users, their performance for simple static method calls is at Darwin's native speed when using GNUstep.
Among the applications they have been evaluating but yet to be working include LLVM-GCC, Midnight Commander, QREncoder, Bayon, The Unarchiver, and World of Warcraft.
More details on this interesting project can be found via the Wiki page. The project is hosted on GitHub with activity every so often and so far Darling has had over 500 commits.