Earlier this year, Blizzard stated that the StarCraft 2 beta would start in "the coming months". They decided to get a little less vague today.

A Blizzard community manager revealed on the Battle.net forums that the beta for the real-time strategy game will begin this summer. As is pointed out in the forum thread, summer begins on June 21st and ends on September 21st (give or take).

Sign-ups for the beta began in early May. You can learn how to register here.