Update: It appears that everyone who registers a Battle.net account is automatically entered in to the beta opt-in for StarCraft II Beta. Only random selected members actually get invited though. Regardless, it's a good idea to check your preferences to make sure you are!
With the with the re-launch of Battle.net recently, Blizzard have also started the StarCraft II Beta opt-in program.
Randomly selected account holders of a Battle.net account will get an invitation to join the anticipated Beta.
Shortly after the new version of Battle.net was launched, fans could create accounts, log on and add games to the account for free download whenever they need it.
Some features are not working perfectly for all regions, but the site is live, and some fans reported getting invitations to the StarCraft II closed beta, which has not yet been given a release date.
For everyone interested, further details are available on the http://www.starcraftwire.net/blog/comments/starcraft-ii-beta-invites-have-started/ network site.