Hmm, they said "as they prepare for the upcoming expansion" its probably just major hardware upgrades, but maybe its more.... Im sure they wont release cata until blizzcon, im sure blizzcon will be the date they announce the cata release date, and what all is gonna be in the collectors edition.

But yeah, 24 hr downtime... ouch. Id almost rather it be 48 hours total, but spread out over a couple weeks or something