If you're planning on picking up a digitally distributed copy of StarCraft II, you can download the game's client right now - you just can't play it.
Hey, in case you hadn't heard, there's a little game called StarCraft II coming out in just under two weeks. I'm pretty psyched for it. But if you're super-duper-ultra psyched, and can't stand to have the game not on your hard drive for even one minute longer, then Blizzard has you covered. You can now download the SC2 client in preparation ... but that's all you can do with it.
See, the game isn't actually out until the 27th, and the client will remain encrypted and (theoretically) impossible to install until then. As soon as the clock hits 10AM CEST (8AM GMT, 3AM EST) on July 27th, you can officially buy and install the game and start killing Zerglings to your heart's content.
Of course, this only applies if you were planning on buying the game digitally from Blizzard rather than getting a boxed, physical copy. For instance, if you've got the Collector's Edition reserved at your local GameStop (like some people *ahem*), this won't do you any good.
This is an interesting reversal from Blizzard, which had previously said that it would wait a short while before selling the game online, giving brick-and-mortar retailers a small window of exclusivity. I guess they just changed their minds.
Of course, considering how Blizzard is still patching the beta, you'll probably have to patch the game up once it goes live regardless of where you purchased it.