XML not generating

Hi
I’m generating a XML file using the following code but the xml only displays when I’m logged in, once I log out of the admin the code no longer works, any ideas? I have a funny feeling it is to do with the id not being passed, is there a way to manually pass this or to use another variable to generate the xml?

<?php
require('./wp-config.php');
header('Content-type: text/xml; charset=utf-8');

function ssp_fetch_images($postid) {
    global $wpdb;
    return $wpdb->get_results("SELECT guid, post_title, post_content, post_excerpt FROM $wpdb->posts WHERE post_parent = $postid AND post_mime_type = 'image/jpeg'");
}

if(isset($_GET['cat'])) {
$cat = $_GET['cat'];
$query = "cat=$cat&showposts=1000";
} else {
$query = "showposts=1000";
} ?>
<gallery>
    <?php $gallery = new WP_Query($query);
    while ($gallery->have_posts()) : $gallery->the_post(); ?>
    <album id="<?php echo htmlspecialchars($post->post_title); ?>" title="<?php echo htmlspecialchars($post->post_title); ?>" description="<?php echo htmlspecialchars($post->post_excerpt); ?>" tn="<?php $key=($post->post_title); echo get_post_meta($post->ID, $key, true); ?>">
        <?php $images = ssp_fetch_images($post->ID);
        foreach($images as $image) { ?>
        <img src="<?php echo $image->guid; ?>" tn="<?php echo $image->guid; ?>" title="<?php echo htmlspecialchars($image->post_title); ?>" caption="<?php echo htmlspecialchars($image->post_title); ?>" />
        <?php } ?>
    </album>
    <?php endwhile; ?>
</gallery>