This week in particular, Twitter this week that customers can trade in a used iPhone
RadioShack will pay up to $200 in store credit for an iPhone 3GS and up to $100 for an iPhone 3G. Note that your trade-in value will depend on your handset's condition so you may get a lesser amount if your phones shows nicks or bruises. A store employee will evaluate your device when you bring it in, but you should be in good shape if your screen isn't cracked and you don't have water damage.
You can apply the amount toward a new iPhone 4 or another RadioShack purchase, or have them put it on a gift card. So just think...if you surrender an iPhone 3GS in pristine condition, you'll be able to walk away with a free 16GB
Check back Monday for Erica Ogg's full story on selling or trading in your old iPhone.