Adding a select all checkbox to your drupal form.

The following snippet can be useful in adding a "select all" box to your drupal forms (5.x - with or without jQuery update).  Note that the code is careful to not check or uncheck any boxes that are not part of the immediate form.
 

//add javascript to the page to handle the select all checkbox
drupal_add_js("function toggleSelectAll(cbx){
  if(\$(cbx).attr('checked') == true){
    \$(\$(cbx).parents('form').get(0)).find('input[@type=checkbox]').each(function(){
      if(!this.checked){
        this.checked = true;
      }
    });
  }
  else{
    \$(\$(cbx).parents('form').get(0)).find('input[@type=checkbox]').each(function(){
      if(this.checked){
        this.checked = false;
      }
    });
  }
}
//Only show the select all box if javascript is working
\$(document).ready(function(){\$('#edit-select-all-0').show();});
",'inline');

//add the select all checkbox to the form
$form['select_all'] = array(
  '#type' => 'checkboxes',
  '#options' => array(0 => "Select All"),
  '#attributes' => array('onchange' => "toggleSelectAll(this);", 'style' => 'display : none'),
);