If you set node creation permission (Node Module on www.yourhost.com/admin/user/permissions) to specific roles, the existing form should take care of this automagically by itself, eg, if the user accessing www.yourhost.com/node/add can't create a specific node type, it won't be presented on that page.
Based upon commentary, node/add is built via:
function node_add_page() {
$item = menu_get_item();
$content = system_admin_menu_block($item);
return theme('node_add_list', $content);
}
/**
* Display the list of available node types for node creation.
*
* @ingroup themeable
*/
function theme_node_add_list($content) {
$output = '';
if ($content) {
$output = '<dl class="node-type-list">';
foreach ($content as $item) {
$output .= '<dt>'. l($item['title'], $item['href'], $item['localized_options']) .'</dt>';
$output .= '<dd>'. filter_xss_admin($item['description']) .'</dd>';
}
$output .= '</dl>';
}
return $output;
}
so you most certainly could create a new YOURTHEME_node_add_list() function that filters the links based upon a user's role realizing that if the user entered the link directly it would nonetheless still work.
You could also create a node-add-list.tpl.php in your theme's directory with basically this in it as a start:
<?php
$output='';
if ($content) {
$output = '<dl class="node-type-list lrfm-ed">';
foreach ($content as $item) {
$output .= '<dt>'. l($item['title'], $item['href'], $item['localized_options']) .'</dt>';
$output .= '<dd>'. filter_xss_admin($item['description']) .'</dd>';
}
$output .= '</dl>';
}
print $output;
and then modify it however you wanted.
Just flush caches after either to make drupal's theme system see and pick up the changes.