Get sub-category to use its parent category template

See:
http://codex.wordpress.org/User:MichaelH/MyPlugins#Use_Parent_Category_Template

I put your code into a filter in functions.php instead and it then works with custom fields:

add_filter('category_template','my_category_template_filter');
function my_category_template_filter(){

	$cat = get_query_var('cat');
    $category = get_category ($cat);

	if ( file_exists(TEMPLATEPATH . '/category-' . $category->cat_ID . '.php') ) {
		$file = TEMPLATEPATH . '/category-' . $category->cat_ID . '.php';
	} elseif ( file_exists(TEMPLATEPATH . '/category-' . $category->category_parent . '.php') ) {
		$file = TEMPLATEPATH . '/category-' . $category->category_parent . '.php';
	}

    return $file;
}

Disclaimer: I have no idea if this is a good idea or not, as I have almost zero idea how actions / filters work. I did see someone filtering the category template for other reasons (to run names instead of ID’s in the file names) so assumed this would work too.

That one also has the custom fields disappearing problem according to the last poster here:

http://wordpress.org/support/topic/230590?replies=12