Hope you still know where yours is!
Attendees of Blizzcon 2007 and the 2008 World Wide Invitational were given keys for a "yet to be announced beta" in their bags of swag. It's now been confirmed that those keys are good for the Wrath of the Lich King beta. From the WoW Europe forums:
We have some information to share regarding the beta codes received by those who attended the 2008 Worldwide Invitational or the 2007 BlizzCon. These codes are going to be used for the Wrath of the Lich King beta test.
If you do have a beta code from one of these events, we urge you to register it immediately and without delay. Full details on how to register your code can be found at the following page:
For more information, please check here.
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