When regularly checking and updating plugins, we don't need persistent update nags bugging clients. Just drop this file in /wp-content/mu-plugins/

hide_wp_update_notifications.php

<?php
/*
 * Hide Wordpress, Plugin, & Theme Update Notifications
 *  
 * When regularly checking for and applying updates, we don't
 * need persistent update nags bugging clients.
 * 
 * This plugin hides the update nags with CSS, but you can still access
 * the updates page list under Dashboard.
 */

function admin_update_nag_css()
{
	?>
	<style>

	/* Hide update arrows in admin bar */
	#wp-admin-bar-updates { display: none !important; }

	/* Hide update notification nags at the top of all admin screens */
	#wpbody .notifications .update, #wpbody .update-nag { display: none !important; }

	</style>
	<?php
}
add_action('admin_head', 'admin_update_nag_css');

?>