Now maybe this is just a dumb cheesehead's point of view...
But why are the devs not making use of the free WoW PTR servers so they can update HB while the patch is being publicly tested (and likely not gonna change much anymore), and thus have a version ready on patchday itself or soon after it (instead of calculating a 7 day downtime AFTER the patch is released).
And to continue on this line of thoughts, I can't imagine that Bossland didn't get any single Cataclysm Beta Invite from one of his 200.000 WoW BOT accounts

Divide those invites and start work on it (if this is not done already, as rumours suggested something along those lines).
imho, the predicted downtime is unneccesary and unneeded with so many options and possibilities to have a "working" version ready at or close after patchday.
Besides all this I just patiently wait for a "stable" release before jumping on the bandwagon again.
Best regards, LiquidAtoR.
In regard to the Auth Servers,
Are you taking EU and US Auth servers offline at the same time?
Since we can play a day longer in the EU...