gistfile1.php

<?php
$post_args = array(
    		'post_type'      => 'post',
			'post_status'    => 'publish',
			'posts_per_page' => 10,
			'meta_query'     => array(
				'relation' => 'OR',
				array(
					'key'     => 'has_been_twittered',
					'value'   => 'no',
					'compare' => '='
				),
				array(
					'key'     => 'has_been_twittered',
					'compare' => 'NOT EXISTS'
				)
			)
		);