The XQuartz project released the 2.1.4 X11 update (requires Mac OS X 10.5.2).

Note: installation requires a reboot. Here are the release notes:

Apple included some of the work done in this project in their 10.5.2 update and will likely include further changes in possible future updates of 10.5.x. It is suggested that you install the latest XQuartz release after updating to 10.5.2 (and any future 10.5.x updates).

10.5.2’s version of X11 is somewhere between the 2.1.1 and 2.1.2 packages released through this site. The launchd mechanism to start X11 in 10.5.2 is unique to Apple’s 10.5.2 and never appeared in a release from this site in its current form (though it was essentially integrated into xinit for 2.1.2).

Installing versions prior to X11 2.1.2 onto OS-X 10.5.2 may work but is not supported.