If you pre-ordered a digital copy of StarCraft II: Heart of the Swarm on Battle.net prior to December 18th, you’ll find that your account is now flagged for beta access. Players that choose to pre-order after the December 18th date can still get in, but they will be admitted in waves on a weekly basis. Certain retailers such as Amazon are also offering limited beta keys as part of a sale promotion.

Heart of the Swarm, the second installment to the StarCraft II is scheduled to launch on March 12th, 2013.

Source: StarCraft II: Heart of the Swarm Beta Announcement