Add text to post list/edit screens?

If you are OK with using javascript, you can try the following in your functions.php file:

add_action('admin_footer', 'my_admin_footer');
function my_admin_footer()
{
    $uri = isset($_SERVER['REQUEST_URI']) ? $_SERVER['REQUEST_URI'] : NULL ;

    $message = NULL;

    if ($uri AND strpos($uri,'edit.php'))
    {
        if (strpos($uri,'post_type=page'))
        {
            $message="My custom message for page list";
        }
        else
        {
            $message="My custom message for post list";
        }
    }

    elseif ($uri AND strpos($uri,'post-new.php'))
    {
        if (strpos($uri,'post_type=page'))
        {
            $message="My custom message for add/edit page";
        }
        else
        {
            $message="My custom message for add/edit post";
        }
    }

    elseif ($uri AND strpos($uri,'post.php'))
    {
        $message="My custom message for add/edit post or page";
    }

    if ($message)
    {
        ?><script>
            jQuery(function($)
            {
                $('<div style="margin-bottom:15px; color:#999;"></div>').text('<?php echo $message; ?>').insertAfter('#wpbody-content .wrap h2:eq(0)');
            });
        </script><?php
    }
}