I guess I will be doing this since Awesome Radical Gaming, a massive powerhouse with it’s 10 (max) readers and completely random updates, isn’t good enough to get one through Blizzard.
You can reserve online and wait 24 hours for the code through email, or go into a store and get the code immediately. The important part is this:
Reserve StarCraft II: Wings of Liberty and receive an invitation to the StarCraft II: Wings of Liberty beta. Available online and in-store while supplies last. Once you receive your Beta Code your order cannot be canceled.