Blizzard started to sell World of Warcraft: Cataclysm via the Battle.net store ahead of the game's December 7 launch date. In other words, gamers may initiate the purchasing process now and will be charged immediately (of course they will).

"Once a player has completed the digital upgrade, he or she will need to log in to World of Warcraft at least once prior to December 7 to begin the background-downloading process and prepare for Deathwing's return."

Here's more info - taken from Blizzard's FAQ:

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? 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.

Blizzard also revealed the system requirements for WoW: Cataclysm not too long ago.