Battle.Net has the news before Blizzard announces it
Word is on Battle.Net that StarCraft 2: Heard of the Swarm is coming on March 12th, 2013. The news comes ahead of any official announcement from Blizzard, who is expected to make an official announcement as soon as today or tomorrow.
Heart of the Swarm, the long-awaited expansion pack, was supposed to be targeted for release late this year; but after beta testing, word was that the game would not make it out this year and instead would arrive next year at some point. Activision had previously announced that the expansion pack would arrive sometime in the first half of 2013, with more specific details to follow.
Battle.Net will be offering a Deluxe Digital Edition of the expansion pack that will include a number of bonus items able to be used in StarCraft 2, World of Warcraft, and Diablo 3, depending on the item.