The StarCraft II Website announces the beginning of beta testing of the Heart of the Swarm expansion for the real-time strategy sequel (thanks BlizzPlanet). They offer a FAQ detailing on how participants are being selected, and the following explanation of what the testing entails:

The Heart of the Swarm beta test has begun! This multiplayer-only test will initially be limited to a select group of pro-gamers, members of the press, Arcade contest winners, and shoutcasters. Beta test participants are not under NDA, so you’re likely going to see live streams of matches, screenshots, and casts of impromptu Heart of the Swarm throwdowns starting . . . mmm, NOW!

We will soon be inviting other players to participate in the beta, so be sure you’ve opted in by clicking on the "Beta Profile Settings" under the Account Management section of your Battle.net account (or just click here). Signing up from a Battle.net account that’s actively playing StarCraft II will help your chances of being selected. Note that opting in doesn't guarantee beta access.

We'll also be working with several fansites and community personalities to provide more opportunities to join the beta test, so keep an eye out here and throughout the StarCraft II community for details as they emerge.

For those who are in the beta already, we appreciate your assistance in helping us test the game. If you’re interested in joining the beta, we wish you luck with the opt-in and (future) giveaways, and hope to see you in a match soon!