How to query_posts that don't have a meta_value set?

You will have to do a custom search query:
http://codex.wordpress.org/Displaying_Posts_Using_a_Custom_Select_Query

they have an example that includes:
<?php

$querystr = “
SELECT wposts.*
FROM $wpdb->posts wposts, $wpdb->postmeta wpostmeta
WHERE wposts.ID = wpostmeta.post_id
AND wpostmeta.meta_key = ‘tag’
AND wpostmeta.meta_value = ’email’
AND wposts.post_status = ‘publish’
AND wposts.post_type = ‘post’
AND wposts.post_date < NOW()
ORDER BY wposts.post_date DESC
“;

$pageposts = $wpdb->get_results($querystr, OBJECT);

?>

and you just change it to:
<?php

$querystr = “
SELECT wposts.*
FROM $wpdb->posts wposts, $wpdb->postmeta wpostmeta
WHERE wposts.ID = wpostmeta.post_id
AND wpostmeta.meta_value = ”
AND wposts.post_status = ‘publish’
AND wposts.post_type = ‘post’
AND wposts.post_date < NOW()
ORDER BY wposts.post_date DESC
“;

$pageposts = $wpdb->get_results($querystr, OBJECT);

?>

That should do it