A guildie posted this on the lemmings forums and he got it from a friend on another server who probably bumped into it at wowinsider by the looks of it:
NOTE: Blizzard has approved use of this method. It is not against the ToS, EULA, or any other acronym.
Pre-orders for the Cataclysm digital upgrade are officially open, and you can start downloading the remainder of the expansion's data the moment your purchase goes through. But what if you're awaiting a Collector's Edition or you don't have the money to upgrade online right now? You don't want to be left waiting for Cataclysm to download and install on launch night.
Thankfully, we've got you covered. There's an easy way to preload all the data you'll need to play Cataclysm the moment it goes live on midnight Dec. 7. Just follow these easy steps:
1. Make sure World of Warcraft and/or the launcher and background downloader are completely closed.
2. Open up your World of Warcraft folder.
3. Open Launcher.wtf in Notepad and change the "accountType" line from LK to CT.
4. Open Config.wtf in Notepad and change the "accountType" line from LK to CT.
Now load up the launcher and you should begin downloading 700 megs or so of new material. That's the last bit of data you need to play on launch night. Enjoy!
Remember: This will only give you the necessary files to play Cataclysm. It will not upgrade your account and you will not be able to access Cataclysm features. You still need to buy the game if you want to play on launch day.
Thanks to Lemon King for the tip!"
I tried it and its downloading another 700mb.
So basically on the 7th of december i wont have to install any game, just upgrade my account with my code, then play away.
Thanks to wow insider