How can I change default X11 application? I compiled my own X11 Server for Mac OS X and want it to replace the original outdated X11.app supplied by Lion. How do I do that?
cd to /usr/X11/bin 
rename the existing X symlink to Xquartz (to X_orig for example) in case you need to revert in the future to the "vanilla" config
symlink X to your compiled binary.