Blizzard now added a Beta-Section to their site.
There, test-willed players with a Battle.net account can sign in and then hope, to be one of the few chosen people to play the StarCraft II beta.
To register for the beta of StarCraft II, you have to create a Battle.net account if you haven't got one. To finally take part in the beta, you need to have a beta profile and the desired preferences. Once the Beta profile, which contains the system specifications and other information, is successfully uploaded, the associated Battle.net account will be added to the pool of potential beta testers for future Blizzard games. This is, as usual, still no guarantee that you are also taken into the beta. A portion of luck may helps.
If you've got any questions, read the FAQs.