I've always thought this would be easier to make than Wine, since OSX is already Unix-like, Darwin is free software, GNUStep already has a lot of the Cocoa base classes and Mac apps use GL instead of D3D. You'd be able to get programs like Photoshop running on Linux with the added bonus of them being able to understand Unix paths.

There's a lot less demand for Mac OS specific applications on Linux than there is for Windows. A lot more people start with Windows and try to move to Linux (bringing their windows applications in wine with them) than start with Mac OS and try to switch. Not to mention the Wine project goes way way back, whereas Mac OS really only started to get popular (again) with Mac OS X.