This post is copied from the forums, here. Go there for player questions, follow-up, etc. Post is by Frejya. The only editing here is for readability with this WP theme.
Beta has begun!!! 
—For those of you wondering, you will need approximately 30GB  available to download and play!—
Thought I’d post this, as there have been a plethora of threads  asking about the “when” and “how” of the beta.  I should note that, with few alterations, this has been copied from the Wrath Beta  thread put forth by Bornakk last year.
It looks as if Battle.net will, indeed, be the method of choice for  opting-in.  See the end of this post for how to use it!
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Q:Where will Blizzard post information about the beta?
A: Blizzard will always post such information on the official forums  and the web site.  Consider any other site and/or e-mail fraudulent.   When in doubt, go to the official website!
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Q: What is an Opt-In?
A: Rather than randomly select from the player base, Blizz uses the  opt-in, where players sign up to play.  This ensures only players who  express an interest in the beta will be selected for participation.
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Q: Who is eligible for the Cataclysm Beta opt-in?
A: Any player who has an active World of Warcraft subscription,  World of Warcraft: The Burning Crusade, and World of Warcraft: Wrath of  the Lich King is eligible. The account must be active when  the beta starts in order to be eligible to receive an invitation.
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Q: How do I know if my account is considered active?
A: You can check the status of your World of Warcraft account at any  time by logging in to your Account Management page.  Once logged in you  will find this information listed under the “Account Information”  section.
Your account will be in one of these states:
– Active: This account is active and can be used for playing.
– Canceled: This account will remain available for play until the  current subscription expires.
– Frozen: This account is inactive and can not be used for playing.
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Q: How do I know if I’ve made it into the beta once I’ve  opted-in?
A: Blizzard will send an e-mail that will direct you to the beta  download site on the Battle.net site.  To be safe, do not follow links  in e-mails – go directly to the site manually!
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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.
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Q: Is this the only opportunity I’ll have to get in the Cataclysm  beta?
A: No. Keep an eye on the official World of Warcraft site and official fansites for more chances.  Wowwiki, Wowhead, and Worldofwar  participated for Wrath.
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Q: How long will opt-in signups be available?
A: Players will be able to opt in until the beta closes.
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Q: If there are multiple World of Warcraft accounts in my  household, can I opt in with each of them?
A: You may, though this doesn’t guarantee every account in the  household will be invited.
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Q: Can I opt in more than once on an single account?
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.
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Q: Are there any character-level requirements to participate?
A: No.
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Q: Are there any specific system requirements I must meet in  order to participate?
A: Short answer: No.  However,  you will be required to download a  program that will send your specs to them (similar to what is contained  in your dxdiag).  This is to ensure that an adequate sampling of all  system types is represented in the testing.
Battle.net Opt-In Procedure :
- 1) go to battle.net
 
- 2) After logging into the Battle.net account, click the Beta Profile  Settings (green) located under Manage My Games.
 
- 3) Create your Beta Profile by downloading the Beta Opt-in  Application.
 
- 4) Save the System Check file for your chosen operating system.  Launch the file to begin loading. Before any system specs will be  updated into your Beta Profile Settings, you must agree to the Blizzard  Beta Test “Opt-in” Agreement.
 
- 5) After selecting Agree, the Battle.net System Check will begin.   Click the yellow Send button to associate your system specs with the  Battle.net account.
 
- 6) Once the system check is completed and sent, you will be taken to  the account management page, where you will be able to view your system  specs.  You can also decide which beta tests you are interested in.   All universes are check-marked by default.  To save any changes that are  made, click the yellow Update Preferences button.
 
What happens once invites get sent:
- 1) You’ll receive an invite in your regular email that is  associated with your battle.net account (make sure it’s current!)
 
- 2) Physically go to battle.net – do not follow any links! This minimizes the chances of you following malicious links in fake  emails!; Log in to your battle.net account
 
- 3) Follow the instructions included in your beta invite email!