Discussion thread on X11 in Leopard with links to updates and info on getting Gimp and Inkscape working

Apple’s X11 Frequently Asked Questions in Leopard (Mac OS X 10.5) – a thread on macosxhints.com forums

Updated Xquartz binaries are available for Leopard; installing the latest of these will fix some issues discovered in Xquartz since Mac OS X 10.5 was released.