You can now pre-order the Cataclysm digital download and Blizzard has released a relevant FAQ. As Mike Morhaime announced at BlizzCon, it is possible to pre-order and download the expansion prior to release and therefore be able to play immediately after launch, servers permitting. Logging into the game after pre-ordering will set your account up to start background downloading Cataclysm data as soon as it is available. Unfortunately, while you may pre-order the physical collector's edition from non-Blizzard sources, the only digital download option is the standard version.
So it looks like I'll be installing my collector's edition while digital download people are dealing with rolling restarts and emergency maintenance. Oh yes, I went there.
The full Cataclysm digital pre-sale FAQ is after the break.