Blizzard has opened up the opt-in process for the upcoming beta for StarCraft II. This manifests itself as a new section in the Battle.net account profile pages called "Beta Profile Settings." In this section you'll download an application that takes some hardware readings, uploads that profile back to Blizzard, and flags you as the type of person who wouldn't mind playing StarCraft II before it comes out.
Of course, there's no guarantee that you'll be selected to participate, and there's no hard start date for the beta. But Blizzard's posted up a FAQ on the process that you should probably read. Especially this part about needing to own games before being able to sign up:
In order for a Battle.net account to be able to opt in to upcoming beta tests, you must have at least one Blizzard Entertainment game that you own attached to the account. Please note that the World of Warcraft trial edition does not count toward this requirement.