Workaround:
        .RadMenu .rmDisabled .rmLink {
            /* IE 8 */
            -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=50)";
            /* IE 5-7 */
            filter: alpha(opacity=50);
        }

        html .RadMenu .rmDisabled .rmLink:before {
            color: #666;
        }