From the Starcraft 2 Wings of Liberty product page on Gamestop:
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.
Gamestop also clarifies that people pre-ordering online will receive an email with the code and instructions within 1 business day, and those buying from stores that offer this will receive the key printed on their receipt.
This is a wonderful opportunity for those who plan on pre-ordering the game to get themselves a beta invite. However "while supplies last" implies that they only have a limited number of keys to offer in this promotion. Be sure to hit the link below to the product page for more info on this promotion, if you plan on pre-ordering online; or contact your local Gamestop to see if the promotion is offered locally.
Links
Gamestop - Starcraft II Wings of Liberty product page
