So this happened:
— StarCraft (@StarCraft) March 15, 2016
What we know so far is that pre-purchasing the DLC pack will get you a unique Nova portrait and a specialized skin for your Ghost units in SC2. In addition, the bundle price that was announced back in December is still in effect, so if you buy all three mission packs together, you’re saving over what buying them individually would cost you.
More details on this as they emerge, but between this, pre-purchasing Whispers of the Old Gods for Hearthstone, buying World of Warcraft Chronicle Vol. 1, and getting a pre-order in for Overwatch, you might as well just sign your tax return over to Blizzard right now.