Can an iPhone still under contract be unlocked?

Heres the deal: AT&T is awful, or at least that has been my families' experience with them. We just opened a plan with Sprint and I got a 4S.

My iPhone 4 is still under contract with AT&T. Our plan is to let the phones get shut off and pay ATT when we can. I'd like to unlock my old iPhone to sell it. I know ATT unlocks iPhones out of contract but I'm sure they won't while within the contact - not without first paying the contract cancellation fees which we (I) don't have at this time.

Is there a way to unlock it otherwise?

was your bill due or already paid for the current bill cycle? Are you still with AT&T?

so can it be unlocked through a crack to restore it to default settings?

And do I need to unlock it if someone who also uses AT&T wants to buy it? Or is unlocking meant for allowing use with any carrier?