Will I be able to use my shipped SC2 activation code on the digital installer?

Once the game activation key is added to your Battle.net account and your game license is active, the option to download the client online will become available.

Please keep in mind, though, that if you try to download the game client early via the digital download option, you will want to make sure that the region of the game client you’re downloading matches the region of the game license that will be shipped to you later on. If your game license region doesn’t match the downloaded client’s region, you will not be able to login.

To clarify further, if a player purchased the North American version of StarCraft II: Wings of Liberty from a store on July 27, he or she would need to ensure that the digital game client downloaded is also from the appropriate region (in this example, from North America). Otherwise, that player would be unable to login until the North American game client is installed.

Esto lo he sacado de Gosugamers.net