jquery.tristate.js is a jquery plugin for adding "tristate" indeterminate functionality to checkboxes in nested lists.
To use it, simply call it on your parent list:
$('ul.tristate').tristate();
Example available here:
http://jlbruno.github.com/jQuery-Tristate-Checkbox-plugin/inqbs/jQuery-Tristate-Checkbox-plugin
A jQuery plugin that allows for checkboxes in nested lists to receive indeterminate state
JavaScript