You remembered correctly, I looked it up after UncleBob mentioned them. You get a $20 Walmart eCard, good for anything on their website (although I guess you can buy it online and have it shipped to a local store, although I know that is not the same).
Best Buy frequently has these deals where you can get 500 Reward Zone points (the equivalent of a $10 gift card) for pre-ordering or buying certain games, nothing for Super Mario Galaxy 2 though.