Seraph-Design

I had the same issue but found a way how to get it done:

$the_cat = get_the_category();
	foreach($the_cat as $category) {
		$category = $category->cat_ID;
		$category = array($category,'');
			foreach($category as $cat){
				if(cat_is_ancestor_of($parentid,$cat)) {
// your code goes in here
								}
			}
}

Just replace $parentid with the ID you want to be the parent.