"As noticed by several users running the developer preview version of OS X 10.8 Mountain Lion, Apple is no longer including its X11 application for running software through the X Window System interface. [...] The X Window System has an extensive open source history, with development on the Mac side being handled under the XQuartz project. Apple's X11 application has been based on this effort, although as with many open source projects bundled for use in commercial software, X11 has generally been somewhat behind the latest XQuartz releases. The current version of X11 is 2.6.3, while XQuartz is currently available in version 2.7.0." Makes perfect sense. I mean, you guys were using XQuartz already anyway, right?