gistfile2.php[345]

function post_is_in_descendant_category($cats, $_post = null) {
	foreach ( (array) $cats as $cat ) {
		// get_term_children() accepts integer ID only
		$descendants = get_term_children( (int) $cat, 'category');
		if ( $descendants && in_category( $descendants, $_post ) )
			return true;
	}
	return false;
}

function is_my_current_cat($id) {   
    if (is_category($id))
        return true;
    if (is_category()) {    
        $descendants = get_term_children( (int) $id, 'category');
        if ( $descendants  )
            foreach($descendants as $cate){
                if(is_category($cate ))
                    return true;
            }
        }
    if (is_single()) {          
        if (in_category($id) || post_is_in_descendant_category($id)) {
            return true;
        }       
    }
    return false;
}

Is it descendant?

A couple helpers to help you figure out if you're within a category in Wordpress. A slight modification of code found here...

http://olykrap.com/post/how-to-check-for-category-descendants/