function wpse24661_filter_wp_title( $title, $separator ) {
    // Globalize $page
    global $page;

    // Determine if current post is paginated
    // and if we're on a page other than Page 1
    if ( $page >= 2 ) {
        // Append $separator Page #
        $title .= ' ' . $separator . ' ' . 'Page ' . $page;
    }
    // Return filtered $title
    return $title;
}
add_filter( 'wp_title', 'wpse24661_filter_wp_title', 10, 2 );