GameStop's got a couple things going on right now for Portal 2 that I wanted to make sure you guys had heard about:

First is the Portal 2 Co-Operative Testing Program running on Kongregate.com – earn badges by collecting Orange and Blue portals by working together with other users. After completing two quests, you’ll receive exclusive rewards. There’s also a good chance there will be cake.
* P-Body’s Quest: http://www.kongregate.com/badge_quests/p-body
* Atlas’ Quest: http://www.kongregate.com/badge_quests/atlas

Second, the pre-order exclusive for Portal 2 is a pair of customized skins for Atlas and P-Body, co-op bots for the game. More info can be found here:
http://gamestop.com/portal2

I'm working with GameStop (hence the username) so definitely let me know if you have any questions. Thanks!

The two first links return a 404, and the pre-order skins aren't that great.