Drupal.checkallAttach = function() {
  // Look for a form input id=checkall - the value of this field is an ID of a
  // container element, where a new checkall checkbox should be inserted.
  $('input#checkall').each(function() {
    $('#' + this.value).each(function() {
      $(this).html('<input type="checkbox" onClick="Drupal.checkAll(checked, this.form)"/>');
    });
  });
}

Drupal.checkAll = function(checked, form) {
  // TODO: just select checkboxes
  $(form.elements).each(function() {
    this.checked = checked;
  });
}

if (Drupal.jsEnabled) {
  $(document).ready(Drupal.checkallAttach);
}