It is an exciting news for players who want to join the World of Warcraft: Wrath of the Lich King Beta. We gathered all information and you will be interested in it.

There are three ways for players to get a beta key. First, Blizzard used "Opt-In" system to send out keys randomly to players who actively opted in, they will uses a "lottery" to assign beta keys from the current opted in accounts.

Blizzard Quote:
I'll touch on this only briefly because this thread is more intended for the Dell questions, but opt-in invites are random--it's a lottery. You won't be sent an email saying "you weren't picked".

The whole point of the opt-ins was that there'd be random lotteries for them as the beta went on. Just because you didn't get your opt-in invite in one wave doesn't mean you won't get into the beta.

We're gonna need lots of testers! Don't worry so much. 
  

Second, from Dell laptops, 5 exclusive beta keys for Lich King were given.

Blizzard Quote:
Folks, if you've had an issue receiving your Dell laptop beta keys, please take the sticky's advice and email wowbetabilling@blizzard.com.

There was indeed some conflicting information, though it was no one's fault in particular. At large we were informed that all Beta Club keys were sent out, and there are understandably people who still believe this is the case, hence my fervent investigation. 
  

Third, From BlizzCon or WWI. BlizzCon or WWI attendees will get your beta keys, and after August 1, BlizzCon and WWI registration codes will no longer be valid for Beta signup.

Blizzard Quote: 
The WWI code replacements aren't going out in waves, they're being addressed case-by-case by the Beta Billing team. They can be reached at wowbetabilling@blizzard.com, or they may be able to assist over the phone as well. 1-800-59-BLIZZ. 
  
After August 1, BlizzCon and WWI registration codes (as in those received at BlizzCon and the WWI) will expire and no longer be valid for Beta signup. The wave post-August 1st, therefore, is the last wave that will incorporate BlizzCon and WWI attendees.