Saw this mentioned elsewhere this morning, didn't remember seeing it mentioned here.
2K Coupon
Good for Civ Rev (all consoles), NBA 2k9 (all + PC), NHL 2k9 (all consoles), Civ IV: Colonization (PC), BioShock (PS3), Carnival Games (Wii & DS), Dora: Snow Princess (Wii & DS), Carnival Games Mini Gold (Wii).
Only at participating retailers though, but the list does included Wal-Mart & GameStop among others.
2K Coupon FAQ