From the Codex

If you are using a custom homepage with custom loops and stuff or a custom front-page, you will have an empty wp_title. Here goes a neat hack to add the description/tagline at the wp_title place on homepage:

add_filter( \'wp_title\', \'baw_hack_wp_title_for_home\' );
function baw_hack_wp_title_for_home( $title )
{
  if( empty( $title ) && ( is_home() || is_front_page() ) ) {
    return __( \'Home\', \'theme_domain\' ) . \' | \' . get_bloginfo( \'description\' );
  }
  return $title;
}