A lot of people give away their old iPhones once they upgrade. Some people just sell them and use the cash to buy the new iPhones. It's a viscious consumer cycle.

When the 3G[s] came out, I actually MADE money. I took my old 3G, and resold it on eBay as jailbroken an unlocked for $525. Now, here's how my mind works –

I'm not including the mothly AT&T charges. I paid $199 for the original $3G. With tax, it came to $223. SAME FOR THE 3G[s]. So TOTAL, for BOTH iPhones and tax, I will have paid $446.

Then I sold my original 3G for $525 as I stated above. That's the cash I used to upgrade to the 3G[s]. So technically, I actually MADE $79 for upgrading.

I gave my wife's old 3G away to my mother, who WAS on T-Mobile. So by me giving away an unlocked phone, she got to keep her #, get a better, premium phone for a cheap-ass plan on a monthly basis. All because I kept my options open.

Not bad, eh?

If you ever choose to sell your iPhone (such as when you need the upgrade to the new iPhone and want the cash) just the fact that your iPhone is unlocked increases the phones value by approx. $200-$250. So for the brand new iPhone 3G[s] 32GB, if it's unlocked on the current OS 3.1.2, your phone is worth approx $700-$800 in places like eBay.

It's basically a way to keep the value of your iPhone increased. And that's if you don't actually use it on other GSM carriers. I'm staying on AT&T too, but I'm running the unlock anyway because I intend to sell next year.