What this says to me is, if any of your clusters have automatic updating turned on, you might get SP3 as a surprise. Since the blog post states that it must be run in attended / UI mode, maybe that is not quite true (it will probably download but not install), but I would check on your clusters just in case. It doesn't sound cut and dried to me.
http://blogs.msdn.com/petersad/archive/2009/03/24/sql-server-2005-sp3-through-microsoft-update-now-supports-clustering.aspx