insert a warning message into post-new.php

function my_admin_notice(){
    global $pagenow;
    if ( $pagenow == 'post-new.php' || $pagenow == 'post.php') {
         echo '<div class="error">
             <p>This notice only appears on the add and edit post & page.</p>
         </div>';
    }
}
add_action('admin_notices', 'my_admin_notice');

http://codex.wordpress.org/Plugin_API/Action_Reference/admin_notices