I had thought about this too. I am sure you can do it, since you would only need to put in your microsim card to make it work. Unless you plan on using T-Mobile, then you would need to cut one down and unlock it. I'm positive that AT&T doesn't bind the phone itself to the purchaser, however you will probably need that person to first activate it.
Also another issue is, the plan that the upgrade is be used on should already on a iPhone type plan. So say you get the phone and they are only using a feature phone from nokia, that plan would still have to pay for the internet and the data plan of an iPhone.
Of course you could always buy a no-commitment phone yourself. Either way, hope that helps.