X11 is no longer included with OS X, but X11 server and client libraries for OS X are available from the XQuartz project: http://xquartz.macosforge.org. You should use the latest available version of XQuartz.
Apple created the XQuartz project as a community effort to further develop and support X11 on OS X. The XQuartz project was originally based on the version of X11 included in Mac OS X v10.5. There have been multiple releases of XQuartz with fixes, support for new OS X features, and additional refinements to the X11 experience. Apple is a contributor to the XQuartz project and has worked to ensure that X11 works as expected in OS X with the latest available versions of XQuartz.