For those of you interested in just buying the digital version directly from Blizzard, you can pre-download the entire game right now. It's a 6.2GB download, so it's a good idea. You don't have to purchase it right now to download it, either. Just download now to beat the rush, and then buy the activation key on the 27th.
http://us.blizzard.com/en-us/games/sc2/