Jason-
You might try doing this:
function ppac_update_post($id) {
delete_post_meta($id, '_anonymous_comments');
// $setting = (isset($_POST["ppac"]) && $_POST["ppac"] == "1") ? 1 : 0;
$setting = 1;
add_post_meta($id, '_anonymous_comments', $setting);
}
I think that should work.
Zach