jQuery Forum

This can be used as a quick workaround with jQuery UI 1.8.2.

.ui-menu .ui-menu-item-disabled {
    margin:0;
    zoom: 1;
    float: left;
    clear: left;
    width: 100%;
    text-decoration:none;
    display:block;
    padding:.2em .4em;
    line-height:1.5;
    color: gray;
}

function disableMenuItem(menuItemId) {
    var menuItem = $("#" + menuItemId);
    menuItem.removeClass("ui-menu-item");
    menuItem.addClass("ui-menu-item-disabled");
    menuItem.children("a").children("div").unwrap();
}

function enableMenuItem(menuItemId, onClickValue) {
    var menuItem = $("#" + menuItemId);
    menuItem.children("div").wrap("<a tabindex=\"-1\" class=\"ui-corner-all\" href=\"#\" onclick=\"" + onClickValue + "\"></a>");
    menuItem.removeClass("ui-menu-item-disabled");
    menuItem.addClass("ui-menu-item");
    menuItem.mouseenter(function(event) {
                menuItem.children("a")
                .addClass("ui-state-hover")
                .attr("id", "ui-active-menuitem");
            })
            .mouseleave(function() {
                menuItem.children("a")
                .removeClass("ui-state-hover")
                .removeAttr("id");
           });
}