Query_posts without a custom field

I’m trying to select posts without a particular custom field. Some posts will have it, some won’t, I want to grab just the ones that do not have it.

Something like this:

$args = array(
    'meta_query' => array(
        array(
            'key' => '$customfield',
            'value' => NULL,
            'compare' => '='
            )
         )
      );

But not that, as that doesn’t work.

Anyone have any ideas?