SQL Server 2005 SP3 through Microsoft Update now supports clusters
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.
Definitely keeping an eye out for our auto-update SQL servers
don't ask why, I don't know why
actually I do know why, but not the 'auto' part