Just after Apple announced the iPhone 3G, Khoi Vinh whipped up a quick graph of the declining value of his iPhone over the past year. He generously estimates that when the iPhone 3G is released in early July, his old iPhone will be worth $100, half of the price for a new iPhone 3G. At the time, I speculated that you'd be hard pressed to find a buyer at $75.
However, the resale market for old iPhones might not be so dismal. AT&T has confirmed to MacWorld that in-store activation of the iPhone 3G will be mandatory:
AT&T spokesman Mark Siegel confirmed for Macworld that activation must be done at the time of purchase, in-store.
For those who want to use their phone on another network, an untethered 8 GB iPhone 3G would cost them at least $374 ($199 + $175 AT&T account cancellation fee). But a cheaper and easier way to get an iPhone that works on T-Mobile, etc. is to buy an old iPhone from an upgrader for $100, maybe even $150?