A little javascript class to help out with tri-state checkboxes.

tristate.js

/**
 * A little "class" to help out with tri state checkboxes.
 */
function TriStateCheckbox(checkbox) {
    var self = this;

    var checkedVal = checkbox.prop("checked");
    var indeterminatePropName = "indeterminate";

    /**
     * Set this to a function value that should be called when the state changes. This event
     * will not be fired in response to programatic state changes (via the checked function)
     */
    this.onTriStateChanged = null;

    /**
     * Get or set the checked state of this checkbox. The checked val can be true, false or
     * null (which represents the indeterminate state)
     * @param {boolean} [val]
     * @return {boolean} iff this function is called with no val argument the current state is returned
     */
    this.checked = function(val) {
        if(typeof val === 'undefined') {
            return checkedVal;
        } else {
            checkedVal = val;
            oldCheckedVal = checkedVal;
            if(val === null) {
                checkbox.prop(indeterminatePropName, true);
                checkbox.prop("checked", false);
            } else {
                checkbox.prop(indeterminatePropName, false);
                checkbox.prop("checked", val);
            }
        }
    };

    var oldCheckedVal = checkedVal;
    var changeFunc = function() {
        checkbox.prop(indeterminatePropName, false);
        checkedVal = checkbox.prop("checked");

        // don't do anything if the value hasn't changed
        if(checkedVal !== oldCheckedVal) {
            oldCheckedVal = checkedVal;

            // notify the listener
            if(self.onTriStateChanged) {
                self.onTriStateChanged();
            }
        }
    };

    checkbox.on("change", changeFunc);
}