Best Buy is doing a one-day promotion on Saturday in which it will give $150 in trade-in credit for a working iPhone 4 or 4S, says CNET. The deal is meant to cover the cost of a new 16GB iPhone 5. Buyers must be eligible for an upgrade (or new contract) with AT&T, Verizon, or Sprint however, and pay any sales tax or carrier activation fees. The promotion is also limited to Best Buy's retail stores.