I think it's the same idea that Wine has for win32 applications, but for OS X applications instead: Provide a binary executable loader and an implementation of all the standard frameworks and libraries, so you can run unmodified mac apps on x86/x64-based Linux.

Edit:

For your Xcode example, in short, yes. (Although - that's probably one of the most difficult apps of them all to bring up, since it integrates and depends on iTunes, mobiledevice/mobilesupport libraries, debugger tools etc.)