Best Buy has noted on its website that between today (Thursday) and October 7th (next Monday), it will be offering $50 gift cards with iPhone 5c purchases. The offer is exclusive to purchases within Best Buy’s physical retail stores via on-location purchases or the in-store pickup option. The gift card will not be applicable to the iPhone 5c purchase, but can be used for future transactions at Best Buy. If you return the iPhone 5c, you must also return the gift card, according to Best Buy. Best Buy is currently not running any promotions for the (still constrained) iPhone 5s.
Update: Best Buy has updated its terms. Yes, you can use the gift card toward the 5c in-store.