Do you need to register to the Closed Beta Opt-In?  You don’t know?  Read the whole FAQ to find out:

Q: Who is eligible for the Wrath of the Lich King Beta opt-in?
A: Any player who has both an active World of Warcraft subscription and World of Warcraft: The Burning Crusade is eligible to opt in to the beta at (https://www.worldofwarcraft.com/account/beta-opt-in.html). The account must be active when the beta starts in order to be eligible to receive an invitation.

Q: Do I need to sign up for the Wrath of the Lich King opt-in if I have an unused beta key from a previous BlizzCon or Blizzard Worldwide Invitational (WWI)?
A: We haven’t announced which betas the BlizzCon or WWI keys will grant access to. Unless you’ve already been notified that you have a guaranteed spot in the Wrath of the Lich King beta, you should opt in if you want a chance to be invited.

Q: What previous contests and promotions will have guaranteed me access to the Wrath of the Lich King beta?
A: Frostmourne Fan Art Contest winners (to be announced in the coming weeks) or players who have purchased the Dell XPS M1730 World of Warcraft Edition notebook are guaranteed beta slots and do not need to opt in to participate in the beta.

Q: If I’m certain I’m already getting a beta invite from another source, will I be able to win another invite if I opt in?
A: Players may opt in to the beta even if they are already guaranteed a spot from another source. In the event you are selected to receive an additional beta invite, it may be gifted to a friend or family member.

Q: Will invites be sent on a first-come, first-served basis, or will invites be sent randomly?
A: Beta invitees will be selected randomly from the entire pool of players who have chosen to opt in. Signing up early doesn’t necessarily mean you’ll be invited sooner than someone who signs up after you.

Q: Does this mean the beta is coming soon?!
A: We are preparing for the beta but do not have an official date to announce just yet. Keep an eye on the official World of Warcraft site for more information.

Q: Is this the only opportunity I’ll have to get in the Wrath of the Lich King beta?
A: No. We will offer other opportunities for players to get in the beta. Keep an eye on the official World of Warcraft site and official fansites for more chances.

Q: How long will opt-in signups be available?
A: Players will be able to opt in until the beta closes.

Q: If there are multiple World of Warcraft accounts in my household, can I opt in with each of them? Or can I only opt in with one?
A: You may opt in to the beta with each account, though this doesn’t guarantee that every account in the household will be invited.

Q: Can I opt in more than once on an account, and if so does that increase my likelihood of being selected?
A: You can only opt in once per account. If you try to opt in a second time, you’ll simply get a message saying that the account has already opted in.

Q: If I receive a beta invite for one account, can I use it for a different account instead?
A: Yes. You will be able to accept the beta invitation on whichever account you like.

Q: Are there any character-level requirements to participate?
A: No, but keep in mind that you will need to have a level-55 character to create a death knight, and we recommend your character be level 68 or higher to play in Northrend.