Kaivax also reminds us that those of us with accounts that are linked to the Mists of Pandaria beta will not be able to download or otherwise access the PTR.
Some forum posters report that the PTR realms are coming up right now. So if you don't have access to the beta but do want to check out what will develop into the final Cataclysm patch, head on over to Battle.net and download the PTR build from your Account Management screen.
All relevant blue posts are after the break.
Editor's Note: The patch 5.0.4 PTR patch notes were just made available, you can find them right here.