With the Heart of the Swarm release quickly approaching, Blizzard has allowed more users into the Heart of the Swarm beta. Any players who pre-purchased HotS on or before December 18 are now flagged for beta access, so if you are one of these people, check to see if you have HotS on your Battle.net account management.
Any players who pre-purchase after December 18 will be allowed into the beta in weekly waves. Be sure to keep checking if you gain beta access!
Battle.net - Heart of the Swarm Beta Access Expanded