Maybe something like this would do?
$cat="About us";
$tax = 'category';
$cat_id = get_cat_ID($cat);
$category = get_term_children($cat_id, $tax);
$pre="";
foreach ($category as $sub)
{
$cat_list .= $pre . $sub;
$pre=", ";
}
query_posts(array( 'cat' => $cat_list, 'posts_per_page' => 1000));
echo '<ul>';
while ( have_posts() ) : the_post();
echo '<li>';
the_title();
echo '</li>';
endwhile;
echo '</ul>';