I have a vague recollection of Stuart McIntyre Tweeting about this when OS X 10.8 Mountain Lion was released, but didn't really follow up at that point.
Having now finally got my act together and installed Mountain Lion, I can see what he means.
On the rare occasion that I want to connect to one of my Unix servers via X11, I'm used to typing the following command: -
$ ssh -X user@hostname.com
which, after a short interval, starts a X11 server, and allows me to log in to the remote box as usual, with the X11 session being established back to the Mac.
Sadly, this is no longer an option.
According to this: -
Apple write: -
X11 is not included with Mountain Lion, but X11 server and client libraries for OS X Mountain Lion are available from the XQuartz project: http://xquartz.macosforge.org. You should use XQuartz version 2.7.2 or later.
So I've headed over to the XQuartz project on MacOSForge, and downloaded a 67 MB DMG file - XQuartz-2.7.4.dmg - and am giving it a try now.
I'll report back ….