Blizzard just announced on Facebook and their website that the first beta keys will be issued soon.
We are preparing to invite testers for the Heart of the Swarm beta very soon! To be clear, this test will be for the multiplayer experience only. We will begin with a select group, including many players who assisted us in testing Patch 1.5, as well as some pro-gamers and press. Over time, we will add more and more testers into the mix.
If you are interested in participating in the beta test, please make sure you are opted in for beta testing. You can do this by clicking "Beta Profile Settings" under the Account Management section of your Battle.net account. Note that the opt-in doesn't guarantee entry, but gives you a chance at joining the beta test. Also, we'll be working with your favorite fansites and community personalities to give away more entries to the beta test, so keep an eye out for invites coming from the greater StarCraft community over the coming weeks.
We can’t wait to get the public test started for Heart of the Swarm, and we look forward to collecting your feedback.
The Swarm is coming!
Source : http://us.battle.net/sc2/en/blog/6983561/