Here is how you can add a CCK node form in a collapsible fieldset into a page callback...
<?php
function MYMODULE_MYFUNCTION() {
$page_contents = "";
// ...code...
$page_contents .= theme('fieldset',
array(
'#title' => 'MYTITLE',
'#collapsible' => TRUE,
'#collapsed' => TRUE,
'#value' => drupal_get_form('MYCCKNODETYPE_node_form', array(
'type'=>'MYCCKNODETYPE',
'uid' => $GLOBALS['user']->uid,
'name' => $GLOBALS['user']->name)
)
)
);
// ...code...
return $page_contents;
}
?>