Where do used iPhones go when they die?

I’m sure I’m not the only person who expected the 4S sales to be anti-climactic, especially following an announcement that many considered a disappointment. But once again, the lines of people waiting to get their hands on the new device were long and the prediction is that it will be yet another record breaking weekend.

Which simply leads me to wonder if this weekend’s buyers are switching to the iPhone from other platforms? Or whether it’s mostly people who absolutely must have the latest and greatest device? And if it’s the latter, what happens to all of those newly retired iPhones?

What’s the point at which the iPhone market gets saturated and there’s no one left to buy them?