Before switcthing to Mac OSX 10.8 everytime I launch application which uses x11 it was starting x11 automatically.
After switching to XQuartz and 10.8 it does not work anymore and I have to launch XQuartz manually.
How to launch XQuartz on demand?
I was having the same problem. Quitting XQuartz and deleting ~/.Xauthority resolved the issue for me. I got the idea to do this from the Mountain Lion, Xquartz, and (lost) autostart thread in the Apple Mailing List. Other suggestions from that thread include:
I don't have to launch xQuartz, I just use Terminal and ssh -Y user@IP. Then X emulation appears to be available as I can launch X windows from the server.