You can override theme('links__system_main_menu', $array)
at the theme level to add "pipes" to the list markup as necessary. I just copied the source of links_system_main_menu and adjusted as necessary:
In page-front.php for example (note I added the #separator value so you could make it anything you want really. The rest of this theme() call is just plain old Zen theme code.
<?php print theme('links__system_main_menu', array(
'links' => $main_menu,
'attributes' => array(
'id' => 'main-menu',
'class' => array('links', 'inline', 'clearfix'),
),
'heading' => array(
'text' => t('Main menu'),
'level' => 'h2',
'class' => array('element-invisible'),
),
'#separator' => '<span class="gold_tassle">|<span>',
)); ?>
Then in my template.php file ...
function MYTHEME_links__system_main_menu($variables) {
$links = $variables['links'];
$attributes = $variables['attributes'];
$heading = $variables['heading'];
global $language_url;
$output = '';
if (count($links) > 0) {
$output = '';
// Treat the heading first if it is present to prepend it to the
// list of links.
if (!empty($heading)) {
if (is_string($heading)) {
// Prepare the array that will be used when the passed heading
// is a string.
$heading = array(
'text' => $heading,
// Set the default level of the heading.
'level' => 'h2',
);
}
$output .= '<' . $heading['level'];
if (!empty($heading['class'])) {
$output .= drupal_attributes(array('class' => $heading['class']));
}
$output .= '>' . check_plain($heading['text']) . '</' . $heading['level'] . '>';
}
$output .= '<ul' . drupal_attributes($attributes) . '>';
$num_links = count($links);
$i = 1;
foreach ($links as $key => $link) {
$class = array($key);
// Add first, last and active classes to the list of links to help out themers.
if ($i == 1) {
$class[] = 'first';
}
if ($i == $num_links) {
$class[] = 'last';
}
if (isset($link['href']) && ($link['href'] == $_GET['q'] || ($link['href'] == '<front>' && drupal_is_front_page()))
&& (empty($link['language']) || $link['language']->language == $language_url->language)) {
$class[] = 'active';
}
$output .= '<li' . drupal_attributes(array('class' => $class)) . '>';
if (isset($link['href'])) {
// Pass in $link as $options, they share the same keys.
$link += array('html' => TRUE);
$title = ($i != $num_links) ? $link['title'].$variables['#separator'] : $link['title'];
$output .= l($title, $link['href'], $link);
}
elseif (!empty($link['title'])) {
// Some links are actually not links, but we wrap these in <span> for adding title and class attributes.
if (empty($link['html'])) {
$link['title'] = check_plain($link['title']);
}
$span_attributes = '';
if (isset($link['attributes'])) {
$span_attributes = drupal_attributes($link['attributes']);
}
$output .= '<span' . $span_attributes . '>' . $link['title'] . '</span>';
}
$i++;
$output .= "</li>\n";
}
$output .= '</ul>';
}
return $output;
}