Blizzard has released a official statement on their World of Warcraft community website, stating that pre-purchasing for the digital copy of the expansion is live and ready.
The Cataclysm looms near, and you can be ready to play the minute the servers go live by pre-purchasing the expansion digitally on Battle.net now. Simply complete the digital pre-sale process on Battle.net, and your account will be flagged for automatic access to Cataclysm when it launches at 12:01 a.m. PST (7:01 p.m. AEDT) on December 7.
Downloading of files will commence once you purchase the copy, and you are required to login at least once to flag your account properly, and begin the download either by playing and having it in the background, or just leaving the initial launcher up.
More details and a Q&A from Blizzard can be found in this press release
The end is soon!