According to Battle.net’s Heart of the Swarm FAQ page, the game will be released on 13 March 2013.
I managed to confirm this myself by navigating to my own Battle.net account management screen. When you choose to pre-purchase Heart of the Swarm, you are taken to a screen that confirms that the game will be playable on 13 March 2012. It also confirms the price: €39.99 (R446).
Currently there is the regular version of the game, as well as the Digital Deluxe Edition which includes extra exclusive content:
- StarCraft II Torrasque Ultralisk and three swarm-infested portraits
- Exclusive Zerg baneling pet for World of Warcraft
- Diablo III Queen of Blades hero wings and Zerg banner sigils
One of the more interesting developments to come out of this is that users have reported that they could purchase the game using their Battle.net account balance. This means that if you own Diablo III, you could raise enough money in the auction house by selling in-game items to purchase Heart of the Swarm.