Enqueue the \'comment-reply\' script.

function wpse88530_enqueue_comment_reply() {
// Enqueue the comment-reply script on 
//single blog post pages with comments 
// open and threaded comments
if ( 
    // Returns the value for the specified option.
    // \'thread_comments\' is a Boolean option where
    // comments are threaded if TRUE, and flat if 
    // FALSE
    get_option( \'thread_comments\' ) 
) { 
    // enqueue the javascript that performs 
    //in-link comment reply fanciness
    wp_enqueue_script( \'comment-reply\' ); 
}
}
// Hook into comment_form_before
add_action( \'comment_form_before\', \'wpse88530_enqueue_comment_reply\' );