Developers testing Apple’s newest cat have confirmed that X11, the X window system that provides a basis for graphical user interfaces (GUIs) and rich input device capability for networked computers, is no longer included with OS X 10.8 Mountain Lion.
Development on the Mac for the X Window System, or X11 as it is commonly referred to, is done under the XQuartz project, which generally outpaces X11 development. Which is why the current version of X11 is 2.6.3, while XQuartz stands tall at version 2.7.0.
Apple seems fed up with this inconsistency, therefore is doing away with X11 in OS X 10.8 Mountain Lion. Users will be prompted to download the open source XQuartz, if the should ever require it.