One of things that long-time Macintosh owners will tell you is that the resell value for Apple computers trends higher than other PC brands. It's not uncommon for even three- or four-year-old Macs to sell for hundreds of dollars, while PCs of similar vintage go for less. It's the sort of thing you tell yourself before you drop a couple thousand dollars on Apple hardware to mute the sting a bit, but in my experience it tends to be true.
I just sold my iPhone 3G—a well-worn 16GB model that was listed with a nice, big picture of the crack in the plastic up around the headphone jack, and isn't unlocked—for $300 on eBay. I'm a bit shocked, frankly. You can pick up the same model brand new at AT&T for $99, or $200 for a 3GS, with a two-year contract.
But for those that don't want to sign up with AT&T, it seems like it's worth paying a few hundred to use an iPhone on another network. That certainly makes the $500 I paid for a new iPhone 3GS feel a lot less painful.
It reminds me an awful lot of what it's like to be a Mac owner who upgrades every 12-18 months. A high initial investment lets me use gear of recent revision, provided I'm willing to keep it in decent shape and resell the old stuff.
And in this case there's an added bonus: I can free an iPhone from the bonds of AT&T service. It's almost like a good deed.