Blizzard Entertainment today announced that StarCraft II: Heart of the Swarm will enter beta very soon - this test will be for the multiplayer experience only. Developers will begin with a select group, including many players who assisted them in testing Patch 1.5, as well as some pro-gamers and press. Over time, they will add more and more testers into the mix.
If you are interested in participating in the beta test, 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.