As visible on the screenshot below, you can now upgrade pre-purchase a digital copy of Heart of the Swarm, which will be available for play on 12.03.2013. We even get to see what the digital collectors edition will offer - goods that are likely to be bonuses in the retail collectors as well - namely: Torrasque Ultralisk unit model, three exclusive portraits, baneling pet for World of WarCraft and Kerrigan wings and banner sigils for Diablo 3.
Additional source: Battle.net Korea