Warn users not to edit the post container

posts-page-warning.php

<?php
// Add to functions.php or place in mu-plugins

// Admin Notice on Posts Page
add_action('admin_head-post.php', 'us2011_postspage_error_notice');
function us2011_postspage_error_notice() {
    $postspage = get_option('page_for_posts');
    if (!empty($postspage))
        add_action('admin_notices', 'us2011_postspage_print_notices');
}
function us2011_postspage_print_notices() {
    $postspage = get_option('page_for_posts');
    // show this only if we're editing the posts page
    if (!empty($postspage) && isset($_GET['action']) && $_GET['action'] == 'edit' && $_GET['post'] == $postspage)
        echo '<div class="error"><p>This page is a container for the most recent posts. It should always be empty, and you should never edit this page. To add a news item, go to <a href="post-new.php">Posts -- Add New</a>.<p></div>';
}
?>