Conditional for determing child pages not working

I’m using this code below on a new site (src is WordPress – see link below). I’ve used it before and it’s worked fine. For some reason it’s not working now for child pages – works for the parent though. The script simply adds a header for the section and all of the pages under it. I feel like I’m missing something obvious. And yes I did assign the children to the correct parent page.

Sample: http://www.greenmountainsoftware.com/wp/about-us/ View any of the children of About Us.

function is_tree( $pid ) {      // $pid = The ID of the page we're looking for pages underneath
    global $post;               // load details about this page

    if ( is_page($pid) )
        return true;            // we're at the page or at a sub page

    $anc = get_post_ancestors( $post->ID );
    foreach ( $anc as $ancestor ) {
        if( is_page() && $ancestor == $pid ) {
            return true;
        }
    }

    return false;  // we arn't at the page, and the page is not an ancestor
}

    if (is_tree('2')) {
?>
    <div id="artwork"><img src="<?php echo THESIS_CUSTOM_FOLDER; ?>/images/about-us-head.jpg" width="980" height="89" alt="About Green Mountain Software" /><img src="<?php echo THESIS_CUSTOM_FOLDER; ?>/images/fadestrip.jpg" width="980" height="18" alt="Green Mountain Software" /></div>
<?php
    }

Code Source: http://codex.wordpress.org/Conditional_Tags#Testing_for_sub-Pages