If you have an earlier iPhone model, you're in luck. Radio Shack will honor their trade-in discounts on top of the sale price. An iPhone 3GS can be worth up to $125 in trade-in value. So if you trade a 3GS in good condition, you could get an iPhone 4 for as little as $25. An iPhone 3G trade-in can be worth up to $75, so you're still getting a great deal.
We aren't sure why there hasn't been more publicity for this deal. iPhone 4 holdouts will find it particularly tempting. You should know that the sale price and trade-ins are only valid in-store. Sale prices end on December 11th, so get moving!
source: RadioShack via Redmond Pie