Also Gamestop is now selling XBLA games and DLC codes in-store. My local one got them in last week. Essentially it's most of the top selling games (Halo, MW 2, Borderlands, Fallout 3, AC 2) that have DLC available, the most recent XBLA games (all of the Summer of Arcade games and 1 or 2 others right before that,) and the EA/THQ online passes for the sports games if you buy the game used.
FYI it's a different card setup as well. No scratch off code or pull to reveal code. Take the card to the register, pay, and the 5x5 code prints on your receipt.