How tot detect is a certain page template is in use.

functions.php

<?php
**/
 * How tot detect is a certain page template is in use.
 * 
 * @link http://www.wupperpiraten.de/2009/12/is-a-certain-page-template-active/
 */
function is_pagetemplate_active( $pagetemplate = '' ) {
    global $wpdb;
    $sql = "select meta_key from $wpdb->postmeta where meta_key like '_wp_page_template' and meta_value like '" . $pagetemplate . "'";
    $result = $wpdb->query( $sql );
    if ( $result ) {
        return TRUE;
    } else {
        return FALSE;
    }
}

@deFries updated snippet, you can see my fork here - https://gist.github.com/Mamaduka/6204839