Solution:
Use following code
add_action('admin_menu','bhhidenag');
function bhhidenag()
{
remove_action( 'admin_notices', 'update_nag', 3 );
}
You can read more about it in the blog-entry where I found this:
http://www.websitesecrets101.com/how-to-hide-wordpresss-update-now-notification