You get the card after you activate an iPhone 5 with a Verizon, AT&T or Sprint on a 2-year plan. You have to activate the phone at Best Buy in order to get the card.
Best Buy is selling the iPhone 5 at the standard $199.99 price for the 16GB model on contract.
If you don’t want a Best Buy gift card, but still want some kind of a discount, RadioShack is slashing prices on iPhones by $20 in store, so that is your alternative.
source: Best Buy via 9to5Mac