function selectAll(formName, checkboxName) {
    var form = document.all.item(formName);
    var elements = form.elements[checkboxName];
    var count = document.getElementsByName(checkboxName).length;
    if (count == 1) {
        if (checkedAll) {
            elements.checked = false;
        } else {
            elements.checked = true;
        }
    } else {
        for (var i = 0; i < elements.length; i++) {
            var e = elements[i];
            if (checkedAll) {
                e.checked = false;
            } else {
                e.checked = true;
            }
        }
    }
    if (checkedAll) {
        checkedAll = false;
    } else {
        checkedAll = true;
    }
}

<s:checkbox name="chk" id="chk" theme="simple"                            onclick="selectAll('moduleForm','cb');"></s:checkbox>

<s:iterator ...>
    <td align="center">
    <s:checkbox name="cb" id="cb" fieldValue="%{pk.code}" theme="simple"></s:checkbox>
    </td>
</s:iterator>