Private by default

<?php
/**
 Plugin Name: Private by Default
 */

add_action( 'post_submitbox_misc_actions' , 'b5f_default_post_visibility' );

function b5f_default_post_visibility(){
    global $pagenow;
    if( 'post-new.php' != $pagenow )
        return;
?>
<script type="text/javascript">
jQuery(document).ready(function($) {
    $('#post-visibility-display').text('Private');
    $('#hidden-post-visibility').val('private');
    $('#visibility-radio-private').attr('checked', true);
});
</script>
<?php
}