Well that explains the Cataclysm digital pre-sale FAQ which was posted earlier today! It's now possible to pre-order the digital version of World of Warcraft: Cataclysm on Battle.net. The expansion will cost the standard price of $39.99, with no current option for a digital Collector's Edition, meaning you may have to either order the box online or wait in line at a store on release day if you want your own Lil' Deathwing.
Simply log in to your account and access the game management for World of Warcraft: Wrath of the Lich King. There should now be a little box on the right side of the page that will allow you to pay for the upgrade and start Cataclysm's download. Once you finish downloading you'll be ready to play at midnight on December 7th when the expansion releases!
And just for completeness sake, check out the FAQ from Slorkuz below.
Q. When will my credit card (or other payment method) be charged?
A. You will be charged on the day that you place your pre-order for World of Warcraft: Cataclysm.
Q. Why do I need to log in to the game once after I’ve upgraded?
A. The first time you log in after pre-ordering, your account will become eligible to begin receiving the Cataclysm data when it’s available. The data will automatically download in the background while you play World of Warcraft or while you have the Launcher window open.
Q. Can I pre-order a physical copy of Cataclysm and have it shipped?
A. Not from Blizzard Entertainment, but many retailers offer pre-orders of physical boxes and Collector’s Editions.
Q. In what regions is the Cataclysm pre-sale available? What day/time is Cataclysm launching in my region?
A. The pre-sale is available in the following regions, with realms scheduled to launch at the following times:
Americas, Australia, New Zealand, and Southeast Asia - December 7 at 12:01 a.m. PST Europe and Russia - December 7 at 12:01 a.m. CET
Q. Will a digital version of the Collector's Edition, with the exclusive pet, be available?
A. No. The World of Warcraft: Cataclysm Collector's Edition is exclusive to physical retail copies. The digital version consists of the standard edition.