Wow, it isn't built on Objective-C? That's completely the reverse of my expectations. I'd picture an "Open Darwin" as a ground-up FOSS rewrite of Mach + XPC, followed by XNU + IOKit, followed by libobjc and CoreFoundation (and libdispatch), followed by most of Cocoa, then all the stuff like LaunchServices to rotate the userland around to an app-bundle-based perspective... and and then maybe getting to the Aqua stuff, sometime around when HURD is usable. You know, basically like the order Mono occurred in.

More to the point, I'd expect the first-milestone goal of an "OpenDarwin" project would be to be able to install and run OSX's Server.app and act as an OpenDirectory master, NetBoot Restore manager, etc. That proves compatibility, and is a useful thing all on its own (because, among other things, it would mean being able to manage OSX from non-Apple hardware†.) Doing a GUI first? Crazy talk!

† ...without spending weeks tearing your hair out trying to conform a Linux LDAP+Kerberos+whatever server to OSX's idea of what an OpenDirectory controller is supposed to look like. I tried, I tried so hard...