I've noticed that the ML documentation makes reference to both orderby and order_by - order_by is the one that works for ML
So possibly the documentation could do with an update. This is the page: http://wordpress.org/plugins/mini-loops/other_notes/
"Order By: What order the posts should be displayed in. Default: date
get_miniloops( array('orderby' => 'date' ) );"
"Here are the acceptable arguments and their default values:
$args = array(
'title' => __( 'Recent Posts', 'mini-loops' ),
'order_by' => 'date',"