Once you have purchased a new device, very little use can be made of your old smartphone. So why not take up the $180 Apple is offering for your old 16GB iPhone 4 and put that towards a brand new iPhone 4S, which costs $199 on contract.
Those who own the white version of the iPhone 4 are in for a better deal, as Apple is willing to offer $200 for it. So essentially you will be paying nothing for a new iPhone.
source - Apple