The Beta should be arriving this summer according to Blizzard president Mike Morhaime:

"This summer we will be opening up external beta testing, and while we haven't announced a release date yet, this phase will also signal the final stretch [of development on the game]," said Morhaime during today's Activision Blizzard conference call."

 You can now apply for the Starcraft 2 Beta here: 

https://us.battle.net/login/login.xml?ref=https://us.battle.net/account/management/beta-profile.xml&app=bam&rhtml=y