The latest update for X11 on Leopard is out - see http://trac.macosforge.org/projects/xquartz here for the release details. This is greatly improved from the version that ships with 10.5 and is a recommended install.

The http://trac.macosforge.org/projects/xquartz/roadmap roadmap suggests that a Tiger version will be available sometime around March 2008.

See http://www.inkscape.org/wiki/index.php/InstallHelp here for more general information.