
Opting in doesn't guarantee access, but here's the basis on which testers are picked: "Beta testers are chosen according to their system specifications and other factors, including recent StarCraft II activity (and an element of luck), when selecting individuals during each round of invitations. Our goal is to have a wide variety of players and system types," explains Blizzard.
The beta focuses on multiplayer gameplay, you don't need to own StarCraft 2 (although it'll improve your chances of access) and there's currently no set ending date.
Head through to the official site for more details.
http://www.computerandvideogames.com...-beta-is-live/