Here is the method.
To play Cataclysm the moment it goes live on midnight Dec. 7 without pre-ordering via battle.net, 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!
And heres a blue okaying it::
Basically you download 7 megs worth of data you would have to load on release night. With this method you just enter your cd key on release night and you're good.
Also this will make you preload Cataclysm login screen.
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.