This code gets 10 posts of all types that are linked to the term_name;

global $wp_query;
query_posts( array(  
    "taxonomy_name" => "term_name", 
    'showposts' => 10 ) 
);

This code gets 10 posts of custom post type "message";

global $wp_query;
query_posts( array(  
    'post_type' => 'message' 
    'showposts' => 10 ) 
);

This code however always ignores the post_type requirement but still selects all post types linked to the term_name;

global $wp_query;
query_posts( array(  
    'post_type' => 'message' ,
    "taxonomy_name" => "term_name",
    'showposts' => 10 ) 
);

I can't see how both can work individually but together they don't unless it might be a bug - any thoughts?